WO1996013970A1

WO1996013970A1 - Algal turf water purification method

Info

Publication number: WO1996013970A1
Application number: PCT/US1995/014317
Authority: WO
Inventors: Walter H. Adey
Original assignee: Ecological Systems, Inc.
Priority date: 1994-11-08
Filing date: 1995-11-08
Publication date: 1996-05-17
Also published as: AU4144896A

Abstract

A method for removing phosphorus and other pollutants from water, comprising the steps of providing a growing surface for algae spores overlain by a thin layer of water, subjecting the spores to light so as to grow an algal turf, adjusting the pH of the water to a level in the range of from about 9 to about 10.5, and haversting a portion of the algal turf after the pollutants have precipitated on the walls of the algae growing thereon.

Description

Algal Turf Water Purification Method

Field of the Invention

The present invention relates to a method for removing pollutants from water. In particular, the present invention relates to a method for purifying water by causing the precipitation of pollutants onto the walls of algal cells growing in the water.

Background of the Invention

Municipal, industrial, and institutional sewage is generally treated today by processes including sedimentation, bacterial action, and chlorination. The net result of such activities for more advanced or tertiary systems is wastewater generally free from particulate organics but still high in nutrients. Moreover, the water may still be contaminated by a variety of pollutants such as heavy metals. Attempts have been made to remove nutrients from waste waters utilizing macro or planktonic algae (Goldman et al., "Inorganic Nitrogen Removal in a Combined Tertiary Treatment Marine Aquaculture System —I. Removal Efficiencies," Water Research, Volume 8, pp. 45-54 (1974)) . Such techniques are directed to scrubbing coupled with control of algal growth, for example, by production of shell fish such as oysters used to remove the algae. Results to date have been mixed indicating more or less efficient nitrogen removal, but only partial success with respect to phosphorus removal.

Algal Turf Scrubbing systems have been developed to remove nutrients and other pollutants from wastewater. These have been patented, as process and equipment, and have been trademarked (ATS™) . My U.S. Patent No. 4,333,263 describes the use of Algal Turf Scrubbing ("ATS") to remove carbon dioxide, nutrients, and other pollutants from wastewaters. My subsequent U.S. Patents Nos. 4,966,096 and 5,097,795 describe equipment for carrying out this function.

Adey et al., "Phosphorus Removal From Natural Waters Using Controlled Algal Production," pp. 29-39, Restoration Ecology (March 1993) , discusses the use of Algal Turf Scrubbing to scrub the nutrient phosphorus from agricultural wastewaters. This paper, and the above-identified ATS patents, concern the role of the ATS process to remove dissolved phosphorus both through (1) metabolic uptake and (2) the physical removal of phosphorus attached to organic particulates. Particulate trapping, i.e., physical removal, is accomplished within the web of algal filaments enhanced by the mucilage production of blue-green algae and diatoms.

A common and primary means for the removal of phosphorus from wastewaters, especially in the tertiary treatment of sewage, but also from agricultural wastewaters, is by chemical precipitation. However, the process of chemical precipitation is extremely expensive.

The development of ATS, in response to the problem relating to pollutants in wastewaters, presented an advance in wastewater treatment. As an illustration of the problem presented by wastewater pollution, it is generally recognized that human and industrial wastes release about three pounds per year of phosphorus per capita to the environment. The release by farming through the fertilization of crops and resultant flushing by rain and irrigation is about equivalent. This presents a very serious pollutant load to the environment, the removal of which is extremely cost sensitive.

Although Algal Turf Scrubbing as described in my U.S. Patent Nos. 4,333,263, 4,966,096, and 5,097,795, which are each hereby incorporated herein by reference in their entirety, provided a significant level of removal of pollutants, additional removal of pollutants, particularly nutrients such as phosphorus, is often desired. The present invention achieves such additional removal of pollutants, including additional removal of nutrients such as phosphorus and also heavy metals, from wastewaters. Another benefit of the invention is the efficient production of nutrient-laden biomass for energy, fertilizer or other uses.

Summary of the Invention

One object of the present invention is to provide a method for removing pollutants from water.

Another object of the present invention is to substantially increase the level of pollution removal that could previously be achieved with ATS.

Another object of this invention is to provide an economical method for removing phosphorus and other nutrients from wastewaters. A further object of the invention is to provide an economical method for removing heavy metals from wastewaters.

Yet another object of the present invention is to provide a nutrient-laden biomass.

To achieve these and other objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the present invention provides a method for removing pollutants from water by providing a growing surface for algae spores below the surface of the water, subjecting the spores to light so as to grow an algal turf, adjusting the pH of the water to a level in the range of from about 9 to about 10.5 thus causing the pollutants to precipitate on the walls of the algae, and harvesting a portion of the algal turf after the pollutants have precipitated onto and/or into the cell walls of the algae growing thereon. According to another embodiment of this invention, in addition to the precipitation of pollutants on the algal cell walls, removal of pollutants by metabolic uptake of the pollutants is enhanced by subjecting the growing surface to water surge motion. Where particulates are present in the wastewater, in a third embodiment the ATS method according to the invention results in pollution precipitation on the algal cell walls, together with particulate trapping and metabolic uptake. According to an especially preferred embodiment, the wastewater contains phosphorus as one of the pollutants targeted for removal according to the method described herein. The invention also provides a nutrient-laden biomass for energy, agricultural hydroseeding, fish feed, and various other uses of the harvested algal turf of the invention. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method particularly pointed out in the written description and claims hereof, as well as the appended drawings.

Brief Description of the Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and together with the description serve to explain the principles of the invention.

FIG. 1 is a schematic drawing of an algal turf in accordance with this invention.

FIG. 2 is a schematic drawing of a microalgae for growing the algal turf in accordance with this invention.

FIG. 3 is a graph showing the effect of pH on phosphorus precipitation on the algal walls in accordance with this invention. FIG. 4 is a schematic illustration of an algal cell having a precipitated pollutant on, and in the pores of, the cell wall.

Description of the Preferred Embodiments

Reference will now be made in detail to the presently preferred embodiments of the invention. The present invention utilizes algal turfs, which are dense mats of small anatomically simple algae generally less than several cm in height. The method for removing pollutants from water according to the preferred embodiment of the present invention comprises the steps of providing a growing surface for algae spores overlain by a thin layer of water, subjecting the spores to light so as to grow an algal turf, adjusting the pH of the water to a level in the range of from about 9 to about 10.5, and harvesting a portion of the algal turf after the pollutants have precipitated onto and/or into the cell walls of the algae growing thereon. The phrase a thin layer of water as used herein shall be understood to mean a layer of between about 2 mm to about 20 cm deep, more preferably about 1-5 cm deep.

The water or wastewater from which pollutants may be removed according to the present invention includes: water from sewage of a wide variety of types and qualities; municipal run-off from, for example, streets and parking lots; mining effluents contaminated with heavy metals and other elements; food processing wastewaters; water from aquaculture control systems that allow closed system operation or control effluents; industrial wastewaters of a broad concentration and type; and agricultural wastewaters of all types, but especially including many diffuse pollution types (as opposed to point sources) that are too costly for current control methods. The invention is, of course, also applicable to point source pollutants.

The pollutants removed from such waters include, for example, phosphorus. When the pH of the water is adjusted to a level in the range of from about 9 to about 10.5 according to the method of the invention, rapid precipitation of calcium phosphate in the form of the mineral hydroxyapatite and/or other organic and inorganic complexes occurs. The precipitation of phosphorus is not into the water column or to the bottom of the operational chamber or basin, but rather onto and/or into the cell walls of the algae forming the algal turf. It is therefore removed when the algae is harvested. As is known from Adey et al. (1993) and Adey and Loveland, Dynamic Aquaria, Academic Press (1991), typical phosphorus metabolic uptake and storage in algae is at values of 0.05% of dry weight for low phosphorus concentrations (in the ambient water) and reaching a maximum of 0.4% at elevated concentrations. In contrast, as Figure 3 shows, at high pH levels according to the invention the dry weight composition of phosphorus ranges upward to about 2% and higher. Accordingly, an algal turf scrubbing system, operated appropriately as described herein to develop elevated and controlled pH, can produce a major additional component for the removal of wastewater contaminants. Precipitation onto and into the cell walls of the algae of an ATS system is thus accomplished. This process applies to a number of contaminant chemical elements, but the removal of phosphorus has specific, strong economic components. In addition to phosphorus, other pollutants that are removed from wastewater according to the method of the present invention include calcium, magnesium, iron, barium, and sulfur and heavy metals including cadmium, chromium, nickel, lead, mercury, copper, and zinc, to name a few. It has long been known that growing algae have the capability of taking up and concentrating many heavy metals (Green and Bedell, 1989) . The algal scrubber process can maintain model ecosystems and wastewaters at acceptably low levels of heavy metals as long as spike additions are not at levels that are toxic to the algae. Other pollutants in the form of elements and compounds that may be precipitated from water according to the present invention are indicated below:

Alkaline Earth Metals

Magnesium, Calcium, Strontium, Barium

Heavy Metals:

Titanium, Chromium, Molybdenum, Nickel, Copper, Zinc, Vanadium, Mercury, Cadmium Metals:

Manganese, Iron, Cobalt, Cobalt, Lead, Boron, Aluminum

Prime Nutrients:

Phosphorus, Sulfur, Carbon The terms "pollutants" or "pollution" herein shall be understood to mean any chemical compounds or elements that are undesirable in wastewater and which are susceptible to precipitation on algal cell walls at elevated pH levels according to the present invention. The phrase "on cell walls" herein shall be understood to mean that the pollutant is precipitated onto the cell walls of the algae and/or into the cell walls of algae cells that are porous in nature, and/or onto and/or into the external cell walls of multilayered algae that may be present in the algal turf.

FIG. 4 shows a schematic drawing of an algal cell of a microalgae. The algal cell consists of a cell wall represented by the width (B) , a cell membrane (11), a lipid/protein (12) embedded in the cell wall, and cellulosic material (13) . As depicted in FIG. 4, whereas in prior ATS processes the removal of pollutants was by metabolic uptake (14) into the algal cell interior, at elevated pH levels according to the present invention, precipitation of pollutants occurs onto (15) and/or into (16) the algal cell wall.

As embodied herein and referring to FIG. 1, the growing surface for the algae consists of any suitable vacant area or substrate in which algae spores may settle. Immediate regrowth of the algal turf will occur if the vacant surface or substrate is sufficiently course to allow a filamentous base of the algae to remain following harvesting.

FIG. 1 shows a schematic drawing of primary algal turf species growing on a growing surface according to the preferred embodiment of the invention, typically a plastic screen. The algal turf species in FIG. 1 include (1) Compsopogon coeruleus, (2) Cladophora crispata, (3) Spirogyra rivularis, (4) Enteromorpha micrococca, (5) Eunotia pectinalis, and (6) Melosira varians, although many others are listed below and in previously cited patents. The very small branched alga attached directly to the screen is Stigeocloni um tenue, while the numerous small ovoid shapes in the algal canopy represent several small pennolean diatoms, particularly Amphora and Cocconeis spp. The algal growth of the listed groups is random on the growing surface, preferably with wave action passing across and through the turf and thus enhancing metabolite cellular-ambient water exchange. The use of a screen, preferably plastic, as a growing surface has achieved optimum results, although other surfaces known in the art can be used. Typically, such a growing surface can be a plastic screen having screen grid dimensions in the range of approximately 0.5 to 5 mm.

Algal turf growth can be achieved in an aqueous environment by providing any suitable vacant area in which spores may settle. The first colonizations are usually microscopic diatoms or blue green algae (cyanobacteria) which are then rapidly dominated by the turf species. In accordance with the present invention, the harvesting of such turfs must occur before they are overgrown in turn by the larger macroalgae. This keeps production rates at a high level and minimizes predation by grazing microorganisms. The rate of harvesting is dependent on light levels, temperature, and surge action. Regrowth of the algal turf will occur if the vacant surface or substrate is sufficiently coarse to allow a filamentous base of the algae to remain following harvesting. Alternately, all algae can be removed and the surface "seeded" with new algal spores. Though this process is slower, it is valuable in special cases, for example, micrograzer control, shut down for repairs, power loss, etc.

Using screens, harvesting can be accomplished by simply scraping the surface or, in the context of artificial growing techniques, the screen can be set up for removal for harvesting. Vacuum harvesting techniques can greatly decrease labor. In addition to the use of screens, other growing surfaces can comprise, for example, any rough surface on which algae can grow.

As used herein, the term "algal turf" and its derivatives refers to a colony of attached microalgae and/or smaller macroalgae and/or spores of the microalgae or smaller macroalgae. The term "microalgae" refers to algae that are smaller than approximately 2 centimeters in height or length. Examples of such algae may be found in U.S. Patent No. 4,333,263, previously incorporated herein by reference. The term "smaller macroalgae" refers to algae that are smaller than approximately 20 centimeters in height or length. Examples of such algae include

Gracilaria (a red algae) , Sargassum, and Dictyota (brown algae) . Benthic microalgae or a colony dominated by such algae are preferred. In certain usage, however, a colony in which a significant percentage or even the majority of the algae are smaller macroalgae may be preferable, particularly where long harvest times are desirable for operational reasons or a coarse diatom-supporting mesh work is desired because of pollution in the form of a high percentage of larger organic particulates.

The present invention utilizes microalgae for growing the algal turf such as depicted in FIG. 2. The microalgae of FIG. 2 is shown attached to a 1mm screen filament (10) with a 2mm mesh (A) and consists of a basal layer (7) attached to a plastic screen, a mid layer or "mucilege" layer (8), and a "canopy" layer (9). Microalgae are anatomically simple, usually less than several cm in height, and belong to all major groups of benthic microalgae. In accordance with the present invention, some prolific groups of algae for low to moderate salinity wastewater use are indicated below:

Cyanophycota (Cyanobacteria) - Blue Green Algae

Oscillatoria, Lyngbya, Schizothrix, Chroococcus Calothrix

Chlorophycota - Green Algae

Ulothrix, Enteromorpha, Spirogyra, Cladophora, Dichotomosiphon, Stigeoclonium, Oedogonium, Mougeotia, Gloeocystis

Chromophycota - (mostly, in this context, Diatoms)

Melosira, Ctenophora, Asterionella, Eunotia, Amphipleura, Cocconeis, Placoneis, Rhoikoneis,

Bacillaria, and others Rhodophycota - Red Algae

Compsopogon

Accordingly, for growing the algal turf, the present invention utilizes major groups of benthic microalgae. Preferably, the benthic microalgae for practicing the present invention are selected from the group consisting of green and blue-green algae for low to moderate saline waters (0-10 ppm) and including red and brown algae for high saline waters. More preferably, they are selected from the group consisting of green and red algae. The microalgae spores for growing such benthic microalgae can be obtained as described in the previously cited patents or can be maintained as described in detail by Adey and Loveland, 1991.

The growing surface is subjected to light so as to promote the growth of the algal turf. Lighting to carry out this process may be either natural or artificial. If artificial light is used, metal halide lighting is most efficient, but fluorescent lamps can also be used as an effective artificial light source. Also, as one in the art would understand, the light wavelength, intensity, and duration can be varied to affect growth of the algal turf and to achieve the growth rate desired.

The use of this invention for pollution scrubbing is dependent on incipient light levels. Studies of the previously known ATS systems have indicated that in the context of a reef microcosm environment, approximately 6 g/m²/day of dry algal biomass can be produced at a light level of 200 μE/m /sec with a nutrient level of 5 μM (N-N0₃=) . 12 g/m²/day of dry algal material have been harvested at light levels of 500 μE/m²/sec and nutrient levels of 1-2 μM (N-N0₃=) . Phosphorus concentration in these harvested algae is about 0.5%. Studies on actual productive reefs indicate that production of dry algal biomass is directly proportional to light intensity at levels up to 1200 μE/m²/sec. At high light intensities and high nutrient levels of wastewater an improvement in the order of levels of 20-40 g(dry) /m²/day occurs. Moreover, when the pH is elevated to and maintained between about pH 9 and pH 10.5, algal harvest production is increased to

50-70g (dry) /m²/day at 2000 μE/m²/sec and the phosphorus level in the harvested turf is increased to about 2%, an increase of up to 300% and higher over the phosphorus removal efficiency attributable to metabolic uptake alone.

In practicing the present invention, the pH of the water is adjusted to and controlled at a level in the range of from about 9.0 to about 10.5. At higher pH levels algal production becomes unduly limited by a lack of available carbon. Preferably, the pH of the water is adjusted to a pH level in the range of from about 9.0 to about 10.0. More preferably, the pH of the water is adjusted to and controlled at a pH level in the range of from about 9.5 to about 10.0. Algal turf scrubbing has previously been performed in a pH range from about 7.0 to about 8.5, usually 8.0 to 8.3, because it was thought that any higher pH would reduce algal production, and hence scrubbing efficiency, due to the lack of available carbon. In this pH range, removal of pollutants and minerals occurs through metabolic uptake and physical trapping processes. However, I have surprisingly discovered that when the water pH is adjusted to a level in the range of from about 9.0 to about 10.5, rapid precipitation of calcium phosphate and, for example, iron, sulfur, and magnesium complexes with phosphorus (depending upon the composition of the ambient water) , as well as hydroxies and lipid complexes, occurs onto and/or into the cell walls of the algae. Further, at these elevated pH levels not only phosphorus is removed, but also, depending on the components of the water, calcium, magnesium, iron, barium, and sulfur, as well as a host of heavy metals which can be more economically removed from the wastewater because of the increase in concentration in or on the removed algae, as compared to strict metabolic removal and particulate trapping.

The adjustment of pH to a level in the range of from about 9.0 to about 10.5 can be accomplished by the management of various parameters. To adjust the pH, preferably light duration is varied by, for example, reducing or increasing the light period from 12 hours to 8 or 9 hours per day. Also, the intensity of light can be raised or lowered and the flow rate may be varied by using, for example, an adjustable flow pump. The pH can also be adjusted by varying water surge rate and intensity, such as by using a dump scrubber (U.S. Patent No. 4,966,096 and Adey and Loveland, 1991), and by varying the algal turf harvest rate. For example, reducing flow increases pH, increasing light increases pH, while increasing surge first increases and then lowers pH. Also, the buffering effect of water salinity and closed system volume is important to pH. The pH of the water can also be adjusted by contacting the water with a gas containing carbon dioxide. Such gases containing carbon dioxide can be derived from, for example, stack gases. Preferably, the gas contains a high percentage by volume of carbon dioxide and very minimum contaminants for most efficient water quality improvement. More preferably, the gas is substantially pure carbon dioxide, although waste gases including carbon dioxide can be used if this is a secondary objective, i.e., scrubbing of C0₂ from stack gases. Additionally, after the precipitation of pollutants, the pH of the water can then be readjusted by passing the water through a lime clarifier, if a low pH effluent is desired. Similarly, in a simple, short term storage environment ATS effluent will drop pH to 7.0 to 8.0 (depending upon salinity) by uptake of atmospheric C0₂.

Another embodiment of this invention involves subjecting the growing surface to water surge to enhance the exchange of metabolites between algal cells of the types of algae listed above and the water media. The combination of attached algal turfs utilizing such simple algae wherein nearly every cell is photosynthetic with water surge is important for metabolite cellular-ambient water exchange and, thus, optimization of such water surge is generally desired. In the absence of wave action and/or water surge, a drop in turf photosynthesis occurs because the wave surge boosts the efficiency of the photosynthetic mechanisms by serving as a small scale mixing agent and by light "flashing." Algal turfs do not light-saturate at normal levels of solar energy. Rather, algal turfs are "sun plants" and can use all the sunlight energy they can get, though there may be a small reduction due to ultraviolet effects at depths less than 20-30 cm under tropical sun. Thus surge action enhances metabolic uptake by increasing algal production. Where the desire for precipitation exceeds the need for metabolic uptake, the surge action may be reduced to zero. In a preferred embodiment, the precipitation and metabolic uptake means of pollutant removal are both employed and thus surge action is generally desired.

Water surge, for example oscillatory water motion, can be obtained in a variety of ways. In an oceanic environment, oscillatory water action is a function of wave motion and may or may not be controlled by attached devices. In the context of mechanical wave generators, oscillatory water surge together with flow rate can be readily controlled. Water surge can be also be created by moving the growing surface relative to the water.

According to the present invention, a portion of the algal turf is harvested after pollutants, such as phosphorus, have precipitated onto the walls of the algae forming the algal turf. The harvested algal turf can contain at least 2.0% phosphorus, for example, per unit dry weight of algae. When phosphorus is precipitated according to the invention, the harvested algal turf can comprise at least 1.5% phosphorus as phosphorus precipitated on the walls of the algae. In addition, if phosphorus in particulates are available, adjustments may be made to algal biomass and/or composition to also trap the particulates, thus increasing the phosphorus content of the harvested algal turf in some situations to about 4%, a significant portion of which is attributable to the site-specific precipitation action provided by the method of the present invention.

In performing the harvesting step, it is preferred that filamentous bases of the algae remain on the growing surface. Harvesting rates are a function of flow rate, screen or platform size, and lighting intensity, as described by Adey and Loveland. Such harvesting can occur at regular intervals in the range of from about one to about two weeks. Generally, care must be taken to prevent the macroalgae from overgrowing the turf or the scrubbing efficiency of the system will decrease significantly. The harvesting interval can be adjusted in accordance with biomass developed to optimize scrubbing rates for particular targeted compounds. For example, long harvest intervals will raise pH and lower biomass production.

When phosphorus, for example, is precipitated according to the present invention, the harvested algal turf preferably comprises at least 1.5% as phosphorus precipitated on the walls of the algae. The concentration of phosphorus in algal turf biomass can then rise to about 4% of dry weight under optimum conditions of high pH and high phosphorus concentration. The actual level is a function of many variables including concentration in the wastewater, the relative particulate versus dissolved concentration of the phosphorus and operational parameters. However, of this maximum amount cited, approximately 0.5% is due to metabolic uptake, 2.0% due to precipitation, and 1.5% due to particulate trapping. Clearly, the precipitation element is extremely important and critical to the efficiency of pollution removal on a large scale. FIG. 3 is a graphical representation of the effect of pH on phosphorus precipitation in accordance with this invention. The results, which were obtained during testing of the present invention at an ATS sewage plant in Patterson, California, show that over the length of the 500 ft ATS utilized, as the mean pH increased within the range of from about 9.27 to about 10.02, the precipitated phosphorus concentration measured as a % of dry algal biomass increased from about 1.61% to about 2.36% while the water concentration of phosphorus decreased from typical levels of between about 4 and 5 mg/1 to between about 0 and 2 mg/1. The pH levels are more critical to phosphorus removal than are ambient concentrations of phosphorus. The present invention is critical to achieve phosphorus removal beyond that which has been heretofore possible.

Current costs of removal of phosphorus by chemical precipitation range from $0.50 to $4.50/gram depending upon concentration. In agricultural amelioration fields today, marsh or STA (storm treatment area) systems are being developed that are regarded as being highly competitive as against chemical methods at low to moderate phosphorus concentrations. These can remove phosphorus at a cost of about $1.00/gram. Large scale ATS systems can remove phosphorus from wastewaters, at least below latitude 40° where sufficient solar energy is available, at a cost of about $0.02/gram. Thus, there has been a considerable economic drive to develop this process by biotechnologic means, and yet the potential as described in this application has not been previously recognized.

It is not only phosphorus that is removed by precipitation onto the walls of the algae. Depending on the components of the particular wastewater, calcium, magnesium, iron, barium and sulfur (as sulfates) can also be similarly precipitated on algal cell walls at elevated pH and thereby removed from the wastewater.

In addition, all algal turfs are capable of removing heavy metals, including cadmium, chromium, nickel, lead, mercury, copper, and zinc from wastewaters by adsorption (ionic linking) into the ionically-charged interstices of cell walls. While some of the heavy metal removal is by metabolic uptake, it is recognized that wall adsorption through uptake by ionic charge characteristics of the wall is primarily responsible. Dead and properly prepared algal cell walls will accomplish this heavy metal removal function, and this is a small but growing industry today despite the fact that costs are very high. The ATS process according to the invention is an inexpensive and considerably more efficient means of accomplishing this critical requirement of some wastewaters.

Most wastewaters are complexes of contaminants. If each contaminant is removed by a separate process, costs are very high. ATS according to the invention is a nearly universal contaminant removal process.

It is contemplated that when practicing the present invention a continuous process can be achieved where the water flows through the algal turf at a rate in the range of from about 100,000 to 300,000 gallons per day or greater. Alternatively, the invention can be practiced as a batch process where the water is recycled a sufficient number of times until the desired level of removal of pollutants is achieved. Contaminated or polluted water is pumped from a storage facility to the improved algal turf scrubber previously described and then returned to the storage facility. After a sufficient number of cycles, the purified water is then pumped to another storage facility.

Another use of the present invention is in the production of biomass. Biomass production is a secondary, but also a very advantageous benefit, of pollution scrubbing of wastewaters at elevated pH levels according to the present invention. The invention provides a method for producing a nutrient-laden biomass comprising the steps of providing a growing surface for algal spores overlain by a thin layer of water, subjecting the spores to light so as to grow an algal turf, adjusting the pH of the water to a pH level in the range of from about 9 to about 10.5, and harvesting at least a portion of the algal turf, preferably at intervals in the range of from about one to about two weeks. In another embodiment of the invention, the production of nutrient-laden biomass is enhanced by subjecting the growing surface to water surge. Additionally, the present invention provides a nutrient-laden biomass produced according to the above method and as described herein.

The above description and drawings are only illustrative of a preferred embodiment which achieves the objects, features, and advantages of the present invention, and it is not intended that the present invention be limited thereto. Any modifications of the present invention which come within the spirit and scope of the following claims is considered part of the present invention.

Claims

I Claim:

1. A method for removing pollutants from water comprising the steps of:

providing algal spores;

providing a growing surface for said algal spores overlain by a thin layer of water;

subjecting said spores to light so as to grow an algal turf;

adjusting the pH of said water to a pH level in the range of from about 9 to about 10.5; and

harvesting a portion of said algal turf after said pollutants have precipitated on the walls of the algae growing thereon.

2. The method of claim 1 further comprising subjecting said growing surface to water surge.

3. The method of claim 3 wherein said water surge is oscillatory and is created by moving said growing surface.

4. The method of claim 1 wherein said pH level is in the range of from about 9.0 to about 10.0.

5. The method of claim 1 wherein said pH level is in the range of from about 9.5 to about 10.

6. The method of claim 1 wherein said pH adjusting step comprises contacting said water with a gas containing carbon dioxide.

7. The method of claim 6 wherein said gas containing carbon dioxide is derived from stack gases.

8. The method of claim 6 wherein said gas is substantially pure carbon dioxide.

9. The method of claim 1 wherein said pH adjusting step comprises altering the intensity or duration of light to which said algal turf is exposed.

10. The method of claim 1 wherein said pH adjusting step comprises altering water surge rate or intensity.

11. The method of claim 1 wherein said pH adjusting step comprises altering the algal turf harvest rate.

12. The method of claim 1 wherein said pollutants are selected from the group consisting of phosphorus, carbon, calcium, magnesium, iron, barium, and sulfur.

13. The method of claim 1 wherein said pollutants comprise phosphorus.

14. The method of claim 13 wherein said pollutants upon precipitation are selected from the group consisting of hydroxyapatite, inorganic and organic complexes containing phosphorus.

15. The method of claim 13 wherein the harvested algal turf contains at least 2.0% total phosphorus per unit dry weight of algae.

16. The method of claim 13 wherein the harvested algal turf comprises at least 1.5% phosphorus as phosphorus precipitated on the walls of said algae.

17. The method of claim 1 wherein in said harvesting step, filamentous bases of said algae remain on said growing surface following harvesting.

18. The method of claim 17 wherein said harvesting occurs at intervals in the range of from about one to about two weeks.

19. The method of claim 1 wherein said algal turf comprises benthic microalgae.

20. The method of claim 19 wherein said benthic microalgae is selected from the group consisting of blue-green algae, green algae, brown algae, and red algae.

21. The method of claim 1 wherein said pollutants comprise heavy metals.

22. The method of claim 21 wherein said heavy metals are selected from the group consisting of cadmium, chromium, nickel, lead, mercury, copper, and zinc.

23. The method of claim 1 wherein said water is selected from the group consisting of sewage, municipal run-off, mining effluents, food processing wastewaters, water from aquaculture control systems, and industrial and agricultural wastewaters.

24. A method for producing biomass comprising the steps of:

providing algal spores;

providing a growing surface for said algal spores overlain by a thin layer of wastewater;

subjecting said spores to light so as to grow an algal turf;

adjusting the pH of said wastewater to a pH level in the range of from about 9 to about 10.5; and

harvesting at least a portion of said algal turf.

25. The method of claim 24 further comprising subjecting said growing surface to water surge.

26. The method of claim 24 wherein said pollutants comprise phosphorus.

27. A biomass produced according to the method of claim 24.