WO1992012785A1

WO1992012785A1 - Effluent treatment

Info

Publication number: WO1992012785A1
Application number: PCT/GB1992/000158
Authority: WO
Inventors: David Arthur Hoare
Original assignee: David Arthur Hoare
Priority date: 1991-01-29
Filing date: 1992-01-28
Publication date: 1992-08-06
Also published as: AU1175692A

Abstract

The invention provides a process for treating effluent consisting of unwanted waste, in particular animal or human domestic waste, suspended in predominantly aqueous liquid. The process comprises first, thoroughly agitating the suspension in an initial mixing tank (11) to effect reduction of the size of larger particles of the suspended waste. The liquid suspension is then passed to a separator device (12) having at least one screen, to effect separation from the suspension of gross solids retained above the screen, the remainder of the suspension, liquid and smaller solids, being passed from beneath the screen to a filtration tank (13) where the suspension is continuously recirculated over a filter material which may be polypropylene. The contents of this tank (13) are then fed to a dissolved air filtration unit (25), where further separation of liquids and solids takes place by flotation and settlement. A floating scum of light solids is removed over a weir (252) from the surface of the liquid suspension held in the dissolved air filtration unit (25) and a sludge of heavier settled solids is removed from the base region of that unit. An output of treated liquid is obtained from a level intermediate the height of the liquid column in this unit (25). At least some of the liquid suspension, before it is fed to the dissolved air filtration unit (25) is saturated with air and held in a chamber (24) where it is maintained under pressure.

Description

EFFLUENT TREATMENT

The present invention relates in particular to a system for effecting reduction of the polluting levels in the combined solid/liquid waste discharge from an agricultural animal rearing unit, the aim being that liquid discharged after having been processed and having been passed through the various sections of the equipment provided for carrying out the process according to the invention, is suitable either for low volume spray application onto farmland as required, or for direct discharge into a water course.

The invention also relates to the treatment of the discharges from the following applications:-

1 ) Dirty water collection systems from cattle units.

2) General liquid/solid storage lagoons and tanks which are containing drainage discharges from the associated livestock housing areas.

3) The treatment of the liquid effluent from the vegetable washing and grading plants. The treatment of the washing liquids and surplus product waste from fruit packaging plants.

4) The treatment of the liquid/solids from poultry processing, cooking and packaging plants.

5) The solid/slurry discharges from an intensive pig holding and fattening units.

6) The treatment of the effluent from dairies and creameries and cheesemaking factories.

7) Industrial and domestic treatment works which have quantities of polluting liquids to handle when the standard existing works are overloaded.

One particular requirement which is of relevance to this invention, is that the process should be particularly effective in the treatment of the combined slurry and wash-down water from a pig fattening unit. One factor is common to all aerobic treatment processes, and that is the production of solids, either in the form of excess activated sludge or humus, for example from a trickling filter operation. The removal of solids, or sludge-disposal, is one of the major problems of sewage treatment and can account for more than half of the total capital and operating costs of the system adopted.

An essential operation in all systems of sludge treatment is dewatering - that is the positive removal of a percentage of the water content in order that the sludge is more suitable for handling for its final disposal. Dewatering methods in current use include vacuum filters, filter presses, centrifuges and open or covered drying beds. Prior to dewatering by mechanical means, the sludges are generally conditioned either physically or chemically in order to render them sufficiently filterable.

It is one object of the process which is the subject of this specification that the filterability of the settlement sludge is increased so as to give positive results under variable conditions in flow rate, sludge levels, temperature and the concentration of solids. The aim of the treatment now proposed, is that it should achieve positive settlement of the suspended solids efficiently and therefore reduce the overall polluting load contained in the liquid flow.

In assessing the design parameters of a treatment plant for installation on an intensive pig farm, there are few strict standards or measurements that can be assumed, as a result for example of a comparison having been made with a treatment plant, of conventional design, for treating the effluent flow from a human population, that is, a flow having a basic domestic loading; and also few effective guidelines are derivable from the design parameters used to design a plant intended for the treatment of a specific industrial waste. The main difference in simplistic terms is that domestic wastes are high volume flows at a relatively low polluting level and in general agricultural wastes are small volume flows at a very high polluting level.

The other basic factor is that while a domestic works handles a flow over a 24 hours, or daily period, that is well defined and is generally constant throughout the year; in contrast, the flow to a treatment system on a pig unit usually varies. This is due to many factors, such as the number of pigs on the unit, the food type and quantity fed to the pigs, the leakage of the fresh water feed system, the washdown of the pig housing pens, surface water draining into the collection tanks and underground sumps.

A standard measurement of the polluting level of a discharged effluent is made in terms of the biochemical oxygen demand (B.O.D.5) which is tested over a period of 5 days at a temperature of 20 °C. The biochemical oxygen demand (B.O.D.5) of domestic sewerage at a flow rate of 30 gallons per head per day (136 litres per head per day), is about 400 mg/litre, (i.e. 0.12 lb (0.55kg)) per head. This falls to about 250mg/litre, after settling out of suspended solids, and is further reduced to a suspended solid level of 20mg/l or less after biological oxidation and final settlement.

Another method of measurement of the polluting level is the permanganate value referred to as the PV value. This is specified as the oxygen absorbed from N/80 acid permanganate in 4 hours at 27 °C. This is a quicker estimation than BOD5 but the relationship to BOD5 varies according to the substance; sucrose, for instance has a high BOD5 but a low PV because its breakdown products, acetic and lactic acids, are fully oxidizable bacteriologically but not by permanganate. The PV of typical domestic sewage is about 100mg/l, falling to about 70mg/l after biological oxidation and final settlement. Research work has given the BOD5 levels of pig effluents in the BOD5 range of 16,000 to 30,000 mg/litre and a daily average flow rate of 1 .6 gallons/head per day (7.5 litres per head per day). Recorded work shows that on a unit of 200 fattening pigs over a period of 56 weeks in a treatment system with separation, high rate filtration and sludge de-watering stages, the overall output of the system consisted of separated solids (19% of slurry input) and de-watered sludge (52% of slurry input).

All systems currently in use, employ the activities of aerobic bacteria. These are heterotrophic micro-organisms which need organic substrates as food to grow either aerobically or anaerobically, and livestock manure is a good food source for many groups of bacteria. Aerobic bacteria (aerobes) require dissolved oxygen for metabolism, using oxygen as a hydrogen acceptor. Anaerobic bacteria use other hydrogen acceptors, such as sulfate and carbon dioxide. Another group of micro-organisms are facultative and they gain energy by either the aerobic or anaerobic pathway. By either route, the bacteria must have available carbon, nitrogen, and a supply of various trace nutrients in the substrate. They must also be in an environment with a satisfactory pH and temperature.

Aerobic treatment for the removal of biodegradable organic matter from liquid wastes is an odourless process and consists of two phases operating simultaneously. One phase is biological oxidation that has by-products such as carbon dioxide and water and it yields energy. The second phase for synthesis of new cells, as shown by the following simplified equation: microbial cells + organic matter + O2 >

CO2 + H2O + NH3 + more cells. Oxygen must be supplied continuously and the amount required depends on the quantity of biological oxygen demand (BOD). Only that fraction of the wastes which has been oxidised can be considered stabilised. The synthesized microbial cells are not in the most stable form. However, they can be settled and separated from the system if desired. Even with long detention times there will be in the system solids residue build-up (sludge) that must ultimately be disposed of.

In a suitable environment (unlimited food supply, sufficient oxygen and so on) aerobic bacteria have a cycle that begins with a period of acclimatisation; then they reproduce at an exponential rate. During this period, the demand for oxygen increases sharply, food substrates (organic wastes) are oxidised, and the mass of cells increases. An important parameter is the food-to-organism ratio (F:M) and this is approximated by the pounds of BOD5 applied per day per pound of volatile suspended solids (VSS) in the treatment plant (lb.BOD5/day/lb.VSS).

The rate of reproduction and oxygen consumption drops if the suitable environment is altered by such factors as depletion of the food or oxygen supply or a build-up of toxic products, or when space for growth diminishes. The final stage of the cycle is endogenous metabolism wherein low levels of oxygen and nutrients are needed. As available nutrients diminish below the level needed for survival, then energy or other factors in the environment become unsatisfactory for maintenance of life, and the cells disintegrate, releasing nutrients for the survivors.

The invention has as one of its main objectives, to provide a treatment process, and equipment for carrying out that process, whereby a cycle can be substantially completed when the system is fed over a 20 to 30 hour period, which is effective to separate from a liquid, waste solids carried in suspension in that liquid. The method and equipment now proposed for the separation of particles of waste material from a liquid in which they are suspended, has the following main stages:-

1 ) separation out of gross solid particles by mechanical means;

2) followed by biochemical and electrostatic treatment of the resultant liquid suspension over a sufficient period of time to dissipate or reduce the attraction of the solids to the liquid in which the solid particles are suspended;

3) then injecting air into some or all of the suspension and maintaining it in a chamber under a pressure of at least 6 bar.

This process results in a liquor which still comprises the solids suspended in the liquid, but which is now characterized by the presence of a mass, if not a near saturation, of small air bubbles. Such a liquor is ready for separation when the pressure is released into its main separate components viz air, liquid and solids.

An important aspect of this invention is the provision of a settling device in which this separation into these three main components can be accomplished.

If required, after being substantially cleansed of solids the liquid is then further treated and filtered by steps known in the art. For example it may be treated biologically, and then preferably, it is passed through an upward flow clarification unit to render it suitable for later use. Means may be provided to promote further solid settlement in these later stages if required.

The invention is further concerned with the provision of initial mixing and agitation of the fresh untreated waste material, to promote reduction of particle size and to achieve a fairly homogenous mixture for subsequent treatment in the later stages of the process.

In one specific aspect of this invention there is provided a process for treating effluent consisting of unwanted waste, in particular animal or human domestic waste, suspended in predominantly aqueous liquid, the process comprising first thoroughly agitating the suspension in an initial mixing tank to effect reduction of the size of larger particles of the suspended waste; then passing the liquid suspension to a separator device having at least one screen, to effect separation from the suspension of gross solids retained above the screen; the remainder of the suspension, liquid and smaller solids, being passed from beneath the screen to a filtration tank wherein the suspension is continuously recirculated over a filter material as herein defined; the contents of this tank then being fed to a dissolved air filtration unit, where further separation of liquids and solids takes place by flotation and settlement; a floating scum of light solids being removed over a weir from the surface of the liquid suspension held in the dissolved air filtration unit and a sludge of heavier settled solids being removed from the base region of that unit; and an output of treated liquid being obtained from a level intermediate the height of the liquid column in this unit; and wherein at least some of the liquid suspension, which is fed to the dissolved air filtration unit, is filled with bubbles of air and fed to a chamber where it is maintained under pressure.

Preferably, the dissolved air filtration unit is disposed at a level spaced beneath the surface of the liquid suspension held in the unit when filled during operation, and an output of treated liquid is derived from this unit at a level at or above the level of the input to the unit, the liquid having to traverse baffle means and sub surface weir means as it travels from said input to said output of the dissolved air filtration unit. δ

The material employed as the filter material will be polypropylene or similar material such that as the liquid suspension is recirculated over it (or as it is moved continuously in the suspensions) it performs an electrostatic conditioning function on the suspension, such that there arises a tendency for the solid particles to lose their attraction for the aqueous molecules, or vice versa. Thus when, at a later stage in the process, air bubbles are introduced into the suspension, the small light suspended particles are more attracted to the air bubbles and rise with these to the liquid surface. At the same time, heavier particles are no longer held floating in the aqueous suspension, and tend to fall to the bottom of the liquid body. A particularly effective filter material has been found to be a random mass of spherelike polypropylene bodies which are each extensively apertured to present a large surface area.

Preferably while the liquid suspension is in the pressure chamber, it is maintained under a pressure of at least 6 bar; and preferably also, heavy, settled solids are periodically removed from the base of this chamber.

In some installations it has been found to be particularly advantageous if all of the liquid suspension which reaches the dissolved air separation unit, is first passed through the pressure chamber being in a condition where it is substantially saturated with air (by which is meant, rendered very full of small air bubbles). A particularly effective separation of sludge from liquid is achieved if this is done. In this way the liquid which reaches the dissolved air filtration unit, which unit operates at atmospheric pressure, is substantially air saturated when first it enters the unit. The dissolved air gives rise to differences in density at various levels in the liquid column. Lighter suspended particles rise with the air bubbles; heavier particles descend. Baffles and weirs are appropriately placed to promote separation of liquids and solids at this stage and the liquid which reaches the outlet as a much cleansed liquor is made to follow an extended tortuous path.

Reverting to the stage where the liquid suspension resides in the filtration unit, preferably the suspension is recirculated over the plastics filtration material for a period of at least twelve hours. The objective here is to achieve in the liquid, an electrostatic charge which is substantially nil. For this purpose the tank is maintained well grounded or electrically earthed. After the said period a measure may be made of the electrostatic potential achieved and when this is substantially nil, and only then, will the liquid suspension be released for treatment in the later stages of the process.

Advantageously all sludges and scums removed or separated from the liquid by the process as so far described are fed to a final dewatering device. This will be equipped with at least one screen for effecting a further separation of solids and liquid.

Optionally, liquid is continuously recirculated from beneath the final dewatering screen to the initial mixing tank or to the filtration tank where the suspension is recirculated over the plastics filtration material. Also, part of the liquid output from the dissolved air filtration unit may be recirculated without further screening to the initial mixer tank or to the filtration tank. Such recirculated liquid may perform a synergistic or priming action, when mixed with and operating upon freshly arriving so far untreated effluent.

Preferably when the suspension is being circulated in the biological filtration unit, the liquid is moved past conductors which are electrically earthed. Preferably most if not all of the metal containers or tanks employed in the process according to the invention are electrically grounded or earthed. One example of a preferred process according to this invention will now be briefly described with reference to the accompanying drawing where there is illustrated, purely diagramatically, one arrangement of plant for carrying out the process.

Referring to the drawing, in a process for treating effluent consisting of animal or human domestic or other waste suspended in predominantly aqueous liquid, this invention provides that incoming effluent suspension, optionally stored in a holding tank 10, is first thoroughly mixed, eg. by pumped circulation, and it may be by mechanical agitation, in an initial mixing tank 1 1 . Relative movement between the liquid suspension and a plastics material may be effected at this stage.

From the tank 1 1 the mixed suspension is passed to a screen separator device 12 having a screen 123 preferably traversed by revolving brushes 125. In this device, solids are removed from above the screen, the remainder of the suspension passing through the screen 123 to settle in a collection compartment 127 in the base of the device 1 2. Hence, the suspension is passed to a filtration tank 13.

In the tank 13, the suspension is continuously circulated over the filter material as above referred to. The contents of the tank are then fed to a dissolved air filtration unit 25. An output of treated liquid is derived from this unit, and may be fed to a storage tank 20. Scum is removed from the surface of the liquid suspension in the unit 25, and heavier settled particles are removed from the base of the unit, both the scum and the heavier particles being fed to a final dewatering screen unit 15. Also fed to the screen of the unit 15, are the gross solids earlier separated out at the separator device 12. Prior to reaching the unit 25, some, or preferably all, of the liquid suspension is fed to a pressure chamber 24 having been just previously supplied with air under pressure. Thus 21 represents a high pressure pump for drawing the liquid from the tank 13 and feeding the liquid under pressure to the pressure chamber 24. 22 represents a pump for supplying air under pressure, to substantially saturate the liquid suspension before it reaches the pressure chamber 24.

It will be noted that in the dissolved air filtration unit 25, the input 251 to the unit is at a level spaced beneath the surface of the liquid suspension held in the unit when filled during operation, this surface level being defined by a weir 252 leading to a chute 253. An output pipe 201 for conveying treated liquid is supplied from the unit 25 by way of a subsurface output weir 254 at a weir level at or above the level of the input 251 to the unit 25. Means are preferably provided whereby the height of the subsurface output weir 254 may be adjusted.

In order to reach the output weir 254, the liquid has to traverse at least one baffle 255. This comprises a plate disposed adjacent the input 251 and extending both below the input and above it, preferably reaching the surface of the liquid head in the unit 25. Although the baffle 255 extends fully across the interior of the unit 25 it does not totally impound the inlet 251 , there being an opening available for liquid movement beneath the baffle. In its lower region the tank constituting unit 25 is tapered towards a bottom sump 256.

In the example of the unit 25 as illustrated, the interior of the unit is divided into two compartments by a central wall 257 which at its upper end terminates just below the liquid surface which latter is at a level defined by the disposition of the weir 252. A second baffle 258 is disposed adjacent the output weir 254. This comprises a plate disposed generally vertically adjacent the weir 251 and extending both below the weir and above it, preferably reaching the surface of the liquid head in the unit 25. Although the baffle 258 extends fully across the interior of the unit 25 it does not totally impound the weir 254, there being an opening available for liquid movement beneath the baffle.

While the liquid suspension is in the pressure chamber 24, it is maintained under a pressure of about 6 to 10 bar; and heavy solids are periodically removed from the base of this chamber. While in chamber 24, the liquid suspension is saturated with air or at least, rendered very full of small air bubbles. The pressure is released as the suspension is passed through a valve 261 , or possibly at a venturi situated just downstream of the valve 261 , this release promoting the formation of small bubbles.

The liquid suspension, saturated with air, or at the least very full of small air bubbles, arrives at the inlet 251 where the pressure is completely released, the interior of the unit 25 and the liquid surface being subject only to ambient pressure. The liquid will contain solids in small particle form, the particles having various weights. The attraction of these particles to the water has been reduced in the earlier stages of the process, particularly as a result of the treatment in the holding and recirculation tank 13. Lighter particles will attach themselves to the small air bubbles which rise to the liquid surface where a scum will form. Heavier particles will begin to fall, eventually settling in the sump 256. The liquid will move beneath the baffle 255, then over the partition wall, then beneath the baffle 258, before it reaches the output weir 254 in the form of "cleansed" liquor. Following this extended tortuous path lighter particles of solids will continue to rise with the air bubbles and the heavier particles will continue to fall to the sump 256. Thus a process of separation, solids from the liquid, continually takes place. At the weir 252 the floating scum is removed (optionally assisted by a skimmer or scraper device 259) from the surface of the liquid suspension held in the dissolved air filtration unit 25; and a sludge of solids is removed from the sump 256 at the base region of the unit, and is periodically released to be fed to the final dewatering unit 15. "Cleansed" liquor passes over the sub surface output weir 254 and is fed to a holding tank 20. From this it may be fed to further plant and subjected to further biological and cleansing treatment.

Reverting to the stage where the liquid suspension resides in the filtration unit 13, preferably the suspension is recirculated over the plastics filtration material for a period of at least twelve hours. The objective here is to achieve in the liquid, an electrostatic charge which is substantially that of distilled water, i.e. nil. For this purpose the tank 13 is maintained well grounded or electrically earthed. After the said period a measure may be made of the electrostatic potential achieved and when this is substantially nil, and only then, will the liquid suspension be released for treatment in the later stages of the process.

The final dewatering screen unit 15 is fed with sludge or scum accompanied by some liquid from a number of sources, namely, mainly large particle sludge from the initial screen separator device 12, sludge in the form of scum from the top surface of liquid in the dissolved air filtration unit 25, and sludge in the form of small particles from the base region of that unit. Also some sludge settles in the base of the pressure chamber 24, and from here it is periodically released and fed to the final dewatering screen unit 1 5.

Sludge in a dewatered state is retained on top of the screen of the unit 15 and this can be removed to another location. Liquid passing through this screen may be continuously recirculated to the initial mixing tank 1 1 or to the tank 13 to be reprocessed through the system. Optionally, part of the liquid output from the dissolved air filtration unit 25 may be recirculated eg. by way of the storage tank 20 without further screening, to the initial mixer tank 1 1 or to the tank 13.

Preferably when the suspension is being circulated in the filtration tank 13, the liquid is moved past conductors which are electrically earthed. Preferably the metal containers of each of the units 12, 13, 24 and 25 at least, are so earthed.

In the plant for carrying out the above process, now to be described in further detail, it will be understood that flow between the various units may be assisted by gravity and that to this end some of the units will in practice be disposed at higher levels than others and that some units may be disposed below ground level. Some or all of the units will have electrical conductors promoting earthing or grounding to release electrostatic charges.

The combined flow of liquid and solids which form the slurry which is to be treated through the equipment is piped to a reception tank 10, e.g. by a system of underground pipes which are laid to give a gravity fall of at least 1 in 40. The flow rate into the tank 10 is controlled by manually operated sluice valves, and an alternative method of some farms is for the slurry to be pumped into a standard tractor-drawn mobile tanker, which then discharges into a reception pit forming the first unit of the system the subject of this invention, namely mixer tank generally designated 1 1 .

Some further details of one preferred example of an initial mixer tank 1 1 are as follows. Tank 1 1 is an open topped, storage tank, the entry part of the feed pipe 1 1 1 is at the top of a vertical wall panel of the tank, the exact position being determined on site to give the required gradient fall to the pipe layout system. The top water level T.W.L. 1 1 2 of tank 1 1 is designed to coincide with the bottom of the input feed pipe 1 1 1 . Installed in tank 1 1 is an electrically driven mixer pump 1 13. This is disposed generally upright; it is secured by horizontal, flat shaped brackets which are bolted to a 'U' channel framework which spans the tank. The cross members of the assembly are supported at each side of the tank. The steel support members are bolted to top edge channel beams that form the top lip of the side panels of the tank walls.

The mixer pump nozzle is advantageously horizontally adjustable and fitted with double joints, 60° vertically adjustable, to give maximum directional flexibility. This pump 1 13 is installed in the tank to thoroughly mix the slurry content of the tank 1 1 before it is pumped to the mechanical separator unit 12 as will be described below. Instead of a pump, any suitable means for thoroughly agitating the liquid suspension in tank 1 1 may be employed.

A feeder pump 1 14 is also placed in the bottom of tank 1 1 . This is a fully submersible electrically driven type, with an open port, with an extended plate and flight wing to give an induced feed to the impellar of the larger solid particles which are to be found in pig slurry. The flexible discharge pipe 1 15 is of a plastic material and is connected from the base of the pump to the inlet pipe of the mechanical brushed screen separator unit 1 2. Some further details of one preferred example of the initial screen separator unit 12 are as follows.

The screen separator unit 12 is mounted on a framework at above ground level. The framework of the screen is constructed from a suitable durable material which preserves the relevant structural strengths. This material can be by way of example, steel, which has had its durability improved by being hot dipped galvanised.

Main elements of the assembly of the separator unit 12, are:- The inlet feed pipe and an inlet weir 122; a screen assembly 123 fed from the weir; a revolving brush assembly 125 movable over the screen; a solids discharge exit point and the gravity discharge chute 126 fed by the revolving brushes from the screen 122; and below the screen, a screened liquid discharge collection compartment 127 with a discharge pipe 128 leading to biological filter tank 13. The unit 12 will also have an overflow pipe together with electrical control equipment and motor means for working the brush assembly.

The incoming slurry after it has been well mixed in the mixer tank 1 1 by the mixer pump 1 13 is pumped to the inlet weir 122 of the brushed screen separator 12 by the feeder pump 1 14 in tank 1 1 . It wells up in an inlet weir chamber and flows over the leading edge of a horizontal weir plate at the end of the screen 123. The liquid falls by gravity through the screen and the solids are retained on the top surface of the screen 123.

The screen 123 consists of two separate sheets of metal closely held together. The lower screen plate is made from a stainless steel type of flat sheet material which has a pattern of small circular holes formed in it, the pattern of the holes is such that the steel screen plate retains its strength and gives the maximum possible free drainage area for the liquid to pass through. Laid on top of the lower screen plate and formed to fit over the complete part circumference of the curved area is an upper screen plate element which is woven from a thin stainless steel wire. The apertures in this woven stainless steel wire screen are less than the diameter of the small circular holes in the lower screen plate, the diameter of the small circular holes in the lower screen plate are dependant on the material being screened and are determined after a series of tests have been undertaken and samples of the slurry to be screened.

The lower and upper screen plates are tightly clamped at both the curved side edges and both the straight ends with a series of recessed self tapping screws, and held firmly to the main framework by brackets. Aluminium strips are used at the straight end to give additional strength to the edges of the woven wire material. This arrangement of the combined screen plates gives a degree of sprung resilience, caused by the brushes passing over the surface area of the screen.

The passing action of brushes 125 traversing the screen 1 23, compresses the wire sections against the rigid section of the lower screen plate, resulting in a vertical lateral thrust action in the weft or the weave of the stainless steel wire, this action causes any particles of solids which have been trapped in the upper screen plate to be dislodged. The solid particles which are retained on the top of the screen plate assembly are swept off the surface of the upper screen plate by a series of separate brush elements which sweep the inner surface of the screen clear and the particles are then directed by the springing action of the individual bristles of the brush element into the top collection area of the solids inclined discharge chute 1 26.

The separate brush elements 125 are mounted on adjustable angle iron holding members and the individual bristles are formed from polypropylene and are tightly packed in a clamped sealing metal edged strip. When the brush bristles wear they are extended radially from the fixed drive assembly arms by two adjustable bolts which are secured by locking nuts to keep the brush bristles pressing firmly against the surface of the screen element. The four separate brush elements are mounted on a radial arm framework which are fixed at 90° to each other from the centre shaft.

The centre shaft is mounted at the centre part of the half circle formed by the screen element. The centre shaft is located in bearings mounted on the top lip of the screen housing tank and a gearbox is fixed at the end of the shaft and secured to the tank by a mounting plate. The gearbox is driven by a standard electric motor, the output shaft is directly coupled to the centre shaft of the brush assembly. The shaft is driven at a pre-determined speed which is calculated after observations of the type and quantity of the solid particles being screened out of the liquid flow.

By way of example it has been found under farm conditions that certain types of pig slurry effluent are adequately dealt with when the centre shaft speed is about 6 r.p.m., and with say four sets of brushes, this then gives an effective performance of a brush sweeping the screen at about 24 times a minute.

The electric motor that drives the screen brush is usefully controlled by a system that is linked to the control circuit of the feeder pump installed in tank 1 1 . This can be controlled either by manual operation or by float level control system which is fitted into tank 1 1 .

The solids which are deposited into the top collection area of the solids discharge chute 126 fall by gravity into the final dewatering screen separator 15, which as will be described in more detail below, may be in the form of one or more trailers, which is/are placed under the discharge chute 126. The top screen 151 of the separator 15 onto which the solids fall from the chute of device 1 2, is a woven mesh of polypropylene. The inductive action of the polypropylene bristles as they sweep the screen element surface of device 1 2, may impose electrostatic conditioning which has a beneficial effect on the dewatering to the solid particles when deposited on the screen 151 stretched across the top of the separator 15.

The liquid which has passed through the upper and lower screen plate of the initial screen separator device 12, is collected in the screened liquid discharge collection compartment 1 27 in the base region of the device, whence it is fed (flow may be by gravity through a plastic pipe arrangement) into the (preferably below-ground and open topped) filtration tank unit 13.

In the biological filtration tank 13 the liquid is continuously pumped and recirculated over a random body of filtration material preferably comprising sphere like bodies of polypropylene. Other materials capable of generating an electrostatic charge may be employed.

Some further details of one preferred example of an initial filtration tank unit 13 are as follows.

The filtration tank 13 is advantageously constructed of sheet metal which is coated inside and out with a protective coating and can be protected from excessive corrosion with the tank being hot dipped galvanised. This tank is earthed into the surrounding ground by a system of earth conductors 131 fixed to a cable connected to a series of copper earth rods and measurements are taken to ensure that a good conductivity factor is obtained. 136 represents a meter gauge for measuring current flow to earth. At a vertical height, of say, for example, 1 .2 metres above the surface floor area of biological filtration tank 13 are fixed a series of horizontal support members 132 which span from one vertical wall of the tank to the opposite vertical wall of the tank. These metal support members are spaced down the length of the tank and welded to brackets welded to the side walls of the tank. Measurements are taken from the support members to ensure that there is a good electrical conductivity to earth obtained.

Placed on the support members fitted in filtration tank 13 are one or more containers 133 built from a durable material which by way of example is iron; the support edge framework is constructed from equal section angle iron, the top, base and four wall sides vertical infill panels are made from metal wire open mesh. The edges of the panels are welded to the angle iron framework on the base and walls and the top panel is tied down on site after filling.

These containers 133 are filled with a plastics filtering medium which comprises synthetic plastics material made e.g. from Polypropylene and constituted for example by sphere like bodies which are extensively apertured and present a large surface area. For example, a filter material has been found to be effective, which has a specific gravity of 0.95 nominal); dimensions in the sphere configuration of 95 x 65 mm., with a surface area of 120m2/m2 and an open voidage of 95%. The filtration material gives a dry nominal weight of 45/g/m3 and an operational weight up to 490/g/m3.

After on site filling of each container 133 with the plastics filtration material, a metal open wire top panel is wired down.

The screened liquid from the brushed screen separator unit 12 flows into the biological filtration tank 13 and is recirculated therein by one or more fully submersible pumps 134 which may be made from a plastic based material. Each such pump is placed on the floor of filtration tank 13 and is secured adjacent to the base of the plastics filter medium filled containers. A plastics feed pipe assembly 135 is fitted at the top of the filter containers. The or each pump 134 is operated continuously and therefore is effectively recirculating the filtered liquid contained in the filtration tank 13 over the plastics filtration material contained within the filter containers 133. Associated with these pump means there may be a level control system operated by a float assembly. This is to prevent any pump running dry if the level of the liquid in the filtration tank 13 drops below the level of the inlet for each pump 134.

Another fully submersible pump may be installed in the filtration tank 13 at the base of the tank and it may be adjacent to the vertical end wall panel. Such a pump may provide an outlet feed frorr, ihe tank 13 to a dissolved air flotation unit 25. However in the piant as shown, feed to the unit 25 is by a high pressure pump 21 which draws liquid suspension from the tank 13 and supplies unit 25 by way of a pressure chamber 24 in which the suspension is maintained at a pressure of say 6 to 10 bar. 22 represents a pumr» device for supplying air to saturate the liquid suspension, already pressurized by pump 21 , before the suspension reaches the unit 25.

26 denotes a pressure relief line whereby the pressure in chamber 23 may be controlled. Liquid flowing through the relief line is reci; : jiated to the tank 13. Pressure in chamber 24 is also controlled by a valve 261 which governs output flow reaching the unit 25. The operation of the valve 261 may be controlled in accordance with the pressure in chamber 24. 27 denotes valve means for limiting and controlling this pressure. 28 denotes a gauge. A venturi choke or throttle may be disposed between the valve 261 and the input 251 of the (DAF) unit 25. A valve 29 in the base of pressure chamber 24 may be periodically opened to release solids. Solids so released are fed to the unit 15.

Further details of one preferred example of a dissolved air filtration (DAF) unit 25 are as follows. The unit 25 is a tank which in plan view is rectangular, but in side view, as shown diagrammatically in the drawing, two opposite sides are inclined to provide a taper towards the bottom of the tank. The inlet pipe 251 and the outlet pipe 201 are both connected to the housing of the DAF tank at a level some 400 mm below the level of the scum weir 252. However the outlet pipe is fed by way of a weir pipe which is adjustable to provide an outlet weir level of some 10 to 100 mm below the level of the scum weir 252.

The baffles 255 and 258 each protrude above the liquid surface level by some 100 mm, and they both extend downwardly through about 1200mm. The container walls in its tapered region exhibit an angle of some 60 degrees between the walls. The central partition 257 extends from the base of the container and terminates some 100 mm below the liquid surface.

The mixture of the saturated water and liquid which contains a proportion of suspended solids enters by a settled stream flow into the main compartment of the DAF unit 25. The microscopic air bubbles attach to some of the suspended solids and form a floating sludge or scum on the top of the main compartment. This is removed by a series of scraper blades of the device 259 which slowly transverse the entire length of the tank compartment and remove the scum. This scum then flows by gravity down the inclined discharge chute 253. This scum which is a combination of fine air bubbles and small particles, falls by gravity or is otherwise fed onto the screen of the final dewatering device 15.

The heavier particles which have been held in suspension quickly fall to the sump 256 at the base of the 60° tapered settlement section at the base of the main compartment of the DAF unit 25. At the base of the settlement area there may be fitted an electrically operated valve which is set on a time switch to discharge at set intervals the solid sludge into the sludge dewatering screen device 15. Thus, the large particle solids discharged from the brushed screen separator 12, the scum and sludges discharged from the upper region of the air flotation unit 25, and the heavier sludges discharged from the sludge settlement tank at the base of the unit 25, are all discharged into the screen device 15, and as will be appreciated, these sludges will contain a liquid component. Also solids released periodically from the base of the pressure chamber 24 will advantageously be fed to the screen of dewatering unit 15.

Accordingly, this device 15 serves as a final liquids/solids separator stage and it is constituted as a sludge dewatering screen unit. Liquid extracted in the device 15 is conveniently re-circulated to the initial mixer tank 1 1 .

Some further details of one preferred example of a final dewatering screen device 15 are as follows.

The unit 1 5 may be constituted either as a static unit equipped with means whereby sludge may be transferred from it; or it may be constituted by one or more wheeled trailer units each adapted to be tractor drawn so that dewatered sludge may be taken to a location where it may be stored or utilized. For serving its separating and dewatering function, a purpose built trailer constituting the unit 15, may have a screen 1 51 of purpose made filter cloth tied over the top edges of the trailer sideboards. When tying the cloth down a little sag is allowed to develop in the centre of the cloth which is also the centre point of the plan view of trailer, this gives a dished effect to retain the solids in. The purpose made filtration screen 151 is made of polymer composition and has an aperture size of 290 microns and is 0.90mm thick with tying ropes round the outer edges.

The solids in the varying sludges are effectively retained by the filter screen 151 and the liquid drains freely through the material without binding. Fitted into the base of the trailer constituting device 15 in this example, is a tank 152 with a stopcock 153 to which is connected a length of flexible plastic pipe 154, which leads to tank 13. This pipe takes the liquid which has filtered through the filter cloth to tank 13. When the screen section of the trailer is full of dewatered solids the pipe is disconnected and the stopcock turned off. The trailer is then taken to a suitable field location and tipped for the disposal of the sludge.

It will be recalled that the invention has as one of its main objectives, to provide a treatment process and equipment for carrying out that process, whereby a bacterial cycle can be substantially completed when the system is fed over a 24 or 25 hour period. A maintenance of one or two milligrams per litre of dissolved oxygen (D.O) in the settled liquid wastes is sufficient to maintain aerobic conditions, the air rate supplied for aerobic digestion being the critical parameter where air is used for micro-organism growth.

The effect of the circulation of the liquid over the plastics filtration medium in tank 13, may be to induce a charge in the solids suspended in the liquid which has had the larger solid particles removed by the brushed wire screen. The walls of the tank 13 being earthed and therefore with a potential difference of nil may well increase the charging capacity of the particles contained with the liquid stream circulating in the tank.

However it is believed that a significant factor is the effect that a biological balance may be produced between the return or recirculated flows of activated liquid and its mixture with the input liquid to the tank 13 which latter has not yet been subjected to biological action. The combination of these two interlinked reactions is to create within biological filtration tank 13 an environment where electronic charges are continuously created. The effect of reducing the specific gravity of the liquid by the introduction of saturated air into the DAF unit 25 causes the sludge to settle and consolidate rapidly. This will be of benefit to give a positive method of reducing the polluting level of a slurry and the liquid discharged from the DAF unit 25 will be reduced considerably in its polluting level, sufficiently to make the low volume irrigation on farmland acceptable. However, if the local site conditions require further treatment of the liquid this can be achieved by feeding the liquid through a system which employs the principles of extended aeration, biological filtration, and an upward flow of clarification with standard sludge settlement, contact stabilization with alternative double filtration.

In particular, the liquid to be treated may flow by gravity from the DAF unit 25 into an adjacent first stage aeration tank which is filled with liquid to a depth say of 1 m. The liquid is then fed to a container filled with plastic random filtration material such as spheres of polypropylene to form a high rate biological filter.

the liquid may then be fed to a sludge settlement tank and then to a second stage which is a duplication of the first stage and therefore the basis of this part of the plant is that the liquid is circulated over independent biological filters.

The flow from the first stage biological filter may be directed into a sludge settling tank which has a tapered base leading down to a sump and the sludge settles and is discharged to a screened trailer. This process is repeated in the second stage of the system.

The liquid from the final sludge settlement tank may be piped for upward flow in clarifier units. These may be of the Banks filter type, which consist of tanks fitted with a suspended layer of small pebbles and gravel, graded with the smaller stones in the lower section of the filter. When liquid flows in an upward direction, suspended solids are trapped in the bed of gravel. The liquid output from the upward flow clarifier system can then be irrigated over adjacent fields or discharged into a watercourse.

By way of explaining the efficacy of the process, a pig unit of say 1000 pigs may be expected to produce a waste slurry output of some 7,500 litres per day. In this instance, and by way of example, the tanks of units 10, 1 1 and 13 will have sufficient capacity, comfortably to hold 7,500 litres each. The chamber 24 will require a capacity of 1 ,500 to 2,000 litres and the unit 25 will have at least a similar capacity. The liquid suspension will be maintained in the treatment tank 13 for a period of at least eight hours, typically 12 to 15 hours, until a charge is registered below a predetermined level - or it may be that current flow to earth is at or below a significant level.

The liquid suspension is then moved through the pressure chamber 24, aerated and pressurized at between 6 and 10 bar and then fed to the dissolved air filtration unit 25 at a rate of about 1 ,000 litres per hour. Following settlement and scum removal in unit 25, the output liquor will be found to have a BOD5 measurement which is only some 10% of the measurement of BOD5 made on the slurry before treatment and it may be as low as 5% or even 3%. In one example tested, slurry initially having a BOD5 measurement of about 12,000 mg/litre, was found, after treatment by the process and apparatus according to the invention, to produce an output of "cleansed" liquid having a BOD5 of less than 350 mg/litre. This result was achieved with 8,000 litres being processed during a 24 hour period.

Claims

1 . A process for the separation of particles of waste material from a liquid in which they are suspended, which has the following main stages:-

1 ) separation out of gross solid particles by mechanical means (12);

2) followed by biochemical and electrostatic treatment of the resultant liquid suspension over a sufficient period of time (in unit 13) to dissipate or reduce the attraction of the solids to the liquid in which the solid particles are suspended;

3) then injecting air into some or all of the suspension and maintaining it in a chamber (24) under a pressure of at least 6 bar.

2. A process for treating effluent consisting of unwanted waste, in particular animal or human domestic waste, suspended in predominantly aqueous liquid, the process comprising

first thoroughly agitating the suspension in an initial mixing tank (1 1 ) to effect reduction of the size of larger particles of the suspended waste; then passing the liquid suspension to a separator device (12) having at least one screen, to effect separation from the suspension of gross solids retained above the screen;

the remainder of the suspension, liquid and smaller solids, being passed from beneath the screen to a filtration tank (13) wherein the suspension is continuously recirculated over a filter material as herein defined;

the contents of this tank (13) then being fed to a dissolved air filtration unit (25), where further separation of liquids and solids takes place by flotation and settlement;

a floating scum of light solids being removed over a weir (252) from the surface of the liquid suspension held in the dissolved air filtration unit (25) and a sludge of heavier settled solids being removed from the base region of that unit; and an output of treated liquid being obtained from a level intermediate the height of the liquid column in this unit (25);

and wherein at least some of the liquid suspension, which is fed to the dissolved air filtration unit (25), is saturated with air and fed to a chamber (24) where it is maintained under pressure.

3. A process according to claim 2 and wherein the input (231 ) to the dissolved air filtration unit (25) is disposed at a level spaced beneath the surface of the liquid suspension held in the unit (25) when filled during operation, and an output of treated liquid is derived from this unit at a level at or above the level of the input (251 ) to the unit, the liquid having to traverse baffle means (255) and sub surface weir means (254) as it travels from said input to said output of the dissolved air filtration unit.

4. A process according to either of claims 2 or 3 and wherein while the liquid suspension is in the pressure chamber (24), it is maintained under a pressure of at least 6 bar.

5. A process according to any one of claims 2 to 4, and in which solids are periodically removed from the base of the pressure chamber (24).

6. A process according to any one or more of the preceding claims, and in which all of the liquid suspension which reaches the dissolved air separation unit (25), is first passed through the pressure chamber (24) being in a condition where it is substantially saturated with air.

7. A process according to any one or more of the preceding claims, and in which, while the liquid suspension resides in the filtration unit (13), the suspension is recirculated over the plastics filtration material for a period of at least eight hours, before being released for treatment in the later stages of the process.

8. A process according to any one or more of the preceding claims, wherein solids in the form of sludge or scum separated out at various stages in the process are fed to a dewatering screen for further separation of liquids and solids.

9. A process according claim 8, and wherein liquid derived from the dewatering screen (151 ) is recirculated and fed to the filtration tank (13).

10. A process according to any one or more of the preceding claims, and wherein at least some of the output liquid from the dissolved air filtration unit (25) is recirculated to the filtration tank (13).

1 1 . A process according to any one or more of the preceding claims, and wherein the plastics filter material is constituted by a random mass of spherelike bodies of polypropylene, each body being extensively finned and apertured so as to present a large surface area.

12. Apparatus for treating a suspension of waste material in an aqueous liquor, in particular an aqueous suspension of animal or human waste, which comprises: a mixing tank (1 1 ) to enhance mixing of the waste and liquor and to encourage reductions in the sizes of the waste particles; a separator (1 2) having at least one screen (123), to effect removal of any gross solids from the suspension; a filtration tank (13) for conditioning the mixed and screened suspension by circulation over a plastics filter material as herein defined; a dissolved air filtration unit (25) for the conditioned suspension, which unit has:

(a) a liquor inlet (251 ) and a liquor outlet at levels beneath the upper surface of the liquor therein;

(b) at least one transverse baffle (255) and sub-surface outlet weir (254) in the liquor flowpath between the said inlet and outlet;

(c) a separation zone above the upper surface of the liquor surface, for the removal of any floating solids forming a scum on the said surface, and

(d) an outlet at or near the base region of the unit, for the removal of settled solids; and a pressure vessel (24) in which at least some of the conditioned suspension is additionally subjected to contact with air or other oxygen-containing gas.

13. Apparatus for carrying out the process as defined in any one or more of claims 1 to 1 1 , and substantially as hereinbefore described with reference to the accompanying drawing.