July 25, 2006 | General

Potential Biomass Production From Recycled Wastewater

BioCycle July 2006, Vol. 47, No. 7, p. 46
Knowing that natural resources are finite but population will continue to grow, we need to develop mechanisms of water reuse technology in every facet of life. Here are lessons to achieve that goal.
Clifford B. Fedler

A GREAT DEAL of research has been conducted successfully on converting biomass to renewable energy as a means to provide long-term sustainable power to replace the nation’s dependence on “nonrenewable” resources. But to produce all this biomass, the principle requirement is energy which comes from the sun and water. There in lies the problem.
Water that could be reserved for human consumption is being diverted to grow these energy crops. To provide a sustainable source of biomass and still reserve fresh water for human consumption, biomass would need to be grown with water recycled from municipal, industrial or agricultural processes that require human “consumable quality” water. An aquatic plant-based system is highly effective in recycling the available nutrients from this recycled water. The overall objective is to obtain biomass production levels of aquatic plants from the recycling of wastewater. Plants under investigation for their potential to improve the quality of recycled water and produce biomass for energy production are duckweed, cattails, water hyacinth and knotgrass. If the major livestock and poultry wastes are converted to energy via biogasification, 34,000 MWe of power could be produced. If that same waste was used to grow water hyacinths for energy, approximately 500,000 MWe could be produced. It is not a matter of if we will utilize reuse technology in the future; it is a matter of when we will use it to sustain economic growth and minimize environmental impact. (See the report in last month’s BioCycle, The Future of Water Reuse by Robert Bastian.)
Water resources available for consumption vary considerably around the world with some areas having more than 26.4 million gallons (100,000 m3) per capita. This abundance in some regions tends to create a perception of an infinite supply with minimal need for conservation. Yet, in other regions of the world, the fresh water available is less than 79,000 gallons (300 m3) per capita. These fresh water resources are being withdrawn in some countries at rates that approach 5,000 m3 per capita, while in most countries the withdrawal is about 500 m3 per capita or less. The world’s water resources are being depleted every year due to the fact that most countries are mining their freshwater resources with withdrawal exceeding the rate of recharge.
Approximately 70 percent of the fresh water consumed worldwide is utilized by food production systems. In the most developed countries of the world, the largest consumer of water is industry, which includes many food-related processes. Only approximately 10 percent of freshwater use is for domestic purposes.
The 1994 animal population summary showed that there were 89.6 million beef cattle, 13.7 million dairy cattle, 60 million swine, 290.8 million laying chickens, 7,017.5 million broilers, and 289 million turkeys in the United States. Within these confined animal feeding operations (CAFO), inefficient consumers are provided with ample amounts of feed and water, which results in production of large quantities of manure. The NRCS estimates that on an as-excreted basis, one animal unit (AU) of beef, dairy, swine, layers, broilers, and turkeys produces 59.1, 80.0, 63.1, 60.5, 80.0, and 43.6 pounds of manure each day, respectively. Table 1 presents an estimate of the annual manure production for selected agricultural animals in the United States. The manure produced by these animals has the potential to cause environmental contamination to groundwater, surface water, and air if improperly managed.
Almost universally, the fresh water that is consumed is utilized once, treated in some fashion, and then discharged. Water withdrawn from our available resources is used for many purposes, but much of that water is used, treated, and then discharged. In the Northern United States, for example, water is drawn from both surface and ground sources. That water is used for domestic purposes and then enters a collection system. All of that water is then diverted to some form of treatment system and eventually discharged into a nearby receiving stream. Once in that stream, the water is essentially sent to the coast and discharged into the ocean. When examining this scenario from the systems approach, water is mined from our freshwater resources and the resulting waste is then discharged into our coastal waters.
The quality of coastal waters is also problematic in this scenario. It was commonly thought that coastal water quality was dependent only upon the discharges that came from local areas surrounding the coast. However, upstream discharge may be an additional contributing factor. Not all of the coastal water quality problems stem from upstream domestic usages because there are many industries discharging their “consumed and treated” water into the same streams. In addition, runoff from agricultural systems also ends up within the same streams. Therefore, solving some of the coastal water quality problems requires serious examination of activities far upstream from the coast.
From a global perspective, water quality means the difference between having an acceptable quality of life and something below international poverty level standards. The most common figure put forth is that over one billion people are without satisfactory access to safe drinking water. Inadequate sanitation is the primary cause for this. In the urban areas of the world, nearly 100 percent of the industrialized nations have adequate sanitation and about 70 percent of the nonindustrial nations have appropriate sanitation available. Adequate sanitation is available in 95 percent of the rural sectors in the industrialized nations, but in only 20 percent of the rural sectors of the nonindustrialized nations. This lack of adequate sanitation is unfathomable considering the technology available to society. In fact, there is sufficient low-tech technology available that there should be no segment of the population without adequate sanitation. The integrated facultative pond is one approach that has tremendous potential for solving this problem.
With our population continuously growing, causing ever-increasing demands upon our natural resources, the future of our water resources will be reduced even further. It is time for a shift in our thinking. All water should be reused to produce biomass and a multitude of valuable products while reducing the demands on our fresh-water resources. For every gallon of water recycled, a gallon of fresh water is reserved for future human consumption. Furthermore, the recycling process itself will contribute to sustained economic development.
From the total amount of animal manure produced, the total dry biomass production is approximately 300 million tons annually. If this manure was processed by recycling it on crop land, approximately one-third to one-half of that crop land would be required if disposed at the current agronomic rates for phosphorous. There are two problems with this potential method of recycling the animal manure. First, if the human population doubles as it is expected to do within the next 50 years or less, then to maintain our current quality of life, the population of animals produced will have to double as well. This automatically rules out the use of crop land for recycling all of the animal manure produced because there will be insufficient land available, even if we do not lose any of the current supply. The second problem with this approach is that many of the animals raised today are in highly concentrated areas. The quantity of manure produced is quite large, usually needing all the crop land available within a large area. Typically, it is thought that if manure is transported more than about 20 miles, the value of the nutrients received for crop production is lost. In many cases where a CAFO exists, there is insufficient land nearby to adequately dispose of the manure in this manner.
Since recycling animal manures by application to crop land cannot be the total solution, alternative methods of recycling need to be considered. If all of this dry manure biomass was converted to a gas in a fluidized bed gasification process and the gas was used to fire a turbine-generator system, over 34,000 MWe of electrical power could be produced annually. This amount of power is valued at approximately $15 billion when sold at $0.05/kWh.
Unquestionably, not all of the manures could be processed in a gasifier because much of it is collected in a liquid form. However, with a solid-liquid separator followed by anaerobic digestion or other processing of the liquid, the solid fraction would be usable in a gasifier. Unfortunately, the total quantity of biomass available for gasification is reduced, but this is where water recycling ties into the system. If the treated water was used to produce other biomass, that material could be utilized in the gasifier or processed into animal feeds.
One example of the multiple-use concept is the recycling of water into various aquatic and terrestrial based products. To allow water to be used multiple times, it is necessary to treat the used water to a level suitable to sustain the development and growth of various products. The level of treatment necessary varies depending upon the level of use of the water and the species of the fish or plants being produced. Each level of treatment of the water can be used to produce another valuable product. Based upon the resources available in a region and the potential products the area market can support, specific processes can be enlisted to recycle output water from one process as the input water for another process to produce marketable products to grow the rural economy.
A modular production system concept can be used to explain how water recycling can be used to integrate businesses in such a way as to provide sustainable development in terms of both economic development, particularly rural development, and a sustainable environment.
Similarly, recycling livestock waste, both the solid and liquid fractions, into various products is an example of a multiple-use concept. To accomplish this task, it is necessary to treat the livestock waste to a level suitable to sustain the growth of the various products. The level of treatment necessary varies greatly depending upon the products produced. In any scenario, each level of treatment of the wastewater can produce another valuable product.
Wastewater generated by livestock, which is extremely high in organic matter contains large quantities of nutrients that are valuable resources for producing various aquatic and terrestrial products. If the wastewater is treated in an anaerobic system (e.g., an anaerobic digester or an integrated facultative pond), the effluent will be high in ammonia-nitrogen. However, a valuable by-product is also produced – methane gas. This ammonia-nitrogen is quite toxic to many species of fish and must, therefore, be either converted or removed. A plant-based system is highly effective in recycling those available nutrients.
Some algae, such as Phormidium boneri and Spirulina can be grown on the treated wastewater effluent and can remove most of the ammonia-nitrogen present in that effluent within less than two days. The algae are a valuable resource as a food for fish or it can be used as a protein source for the livestock that supply the wastewater. In this case, the waste recycled back to the originating animals goes through two process steps before the recycling process occurs, thus eliminating potential biotoxicity or bioaccumulation problems. Since the wastewater may not be suitable, at this point, for the survival of some species of fish, another process step may be required that will lead to the production of even more valuable by-products such as duckweed. Once again, these products can be used as feed for fish or livestock and continue to support sustainable economic growth.
For the fish production process, it is desirable to allow the water to flow through the production facilities continuously as a means of flushing out waste produced by the fish. The effluent water from the previous flow process step is stored in a tank where either algae or more duckweed is grown and harvested. This water is then pumped into the fish tanks while the fish tank effluent flows back into this same storage tank and is recirculated back into the fish tanks. In a properly operating system, this recirculating tank of water supports the growth of algae, which is a highly marketable feed source for fish. In fact, algae are the elements that provide the color to tropical fish, which increases their value substantially in the marketplace.
Tropical fish are not the only fish that can be produced in this type of system. The species tested were koi (Cyprinus carpio), molly (Poecilia latipinna), Platty (Xipmophorus maculatis), Tilapia (Tilapia aurea), Channel catfish (Lctalurus punctatus), Bluegill (Lepomis macrochirus), Fathead minnow (Pimephales promelas), and Redfin shiner (Notropis umbratilis). Each species tested maintained survival rates equivalent to or better than the control tests.
Utilization of the duckweed for animal feeds appears to have benefits that go beyond the value of the protein within the duckweed. Feeding trials conducted with both swine and cattle showed positive benefits when duckweed was included as the protein source vis-à-vis the typical soya or corn based diet. Swine fed the duckweed not only show an increase in average daily gain during a short trial period of 21 days, but they also continued to maintain a higher growth rate even after being removed from the duckweed diet and placed back on the basal diet. For the sheep tested, there was no significant (P In some production systems, an abundance of water flowing through a fish production system provides excess water that can be recycled back into the livestock operation. In view of the fact that fish require a much higher quality of water than cattle and since the water has been treated through several processing steps, the quality is more than sufficient to be used as drinking water for livestock. Another alternative for the discharge water is use on a terrestrial-based crop.
In all the modular production components of a system, there is a certain quantity of biomass that is generated and often requires disposal. Normal disposal in a landfill or similar system is usually expensive and justifies the need to seek an alternative solution. Since the biomass is an organic source, the most likely solution to disposal is the conversion of that biomass to electricity via a gasifier system. When this type of process is utilized, the production system has its own power source and, in many situations, will be able to generate more power than is required by the system. An additional potential benefit is the utilization of the heat normally emitted by an electrical production system. If aquatic plants or fish are being produced, this excess heat can be utilized to maintain a specific temperature for the production of some higher valued products.
Most terrestrial based crops annually produce between 0.7 and 3.5 tons of dry residual biomass per acre with usually a similar amount of grain. Switchgrass has been noted as a potential energy crop because it can produce from 4 to 5 dry tons of biomass per acre annually. In comparison, aquatic plants can annually produce from 2 to 80 MT/ha with southern states on specific crops such as water hyacinths (Eichhornia crassipes) achieving the upper end of that production range. Tests completed in our greenhouse modular production system recycling cattle manure have shown production levels of water hyacinth of about 48 ton/acre (dw) annually. Recent research found water hyacinth production levels of 66 MT/ha (dw). Cattails grown in our system produced about 16 ton/acre whereas other research showed that nearly 24 MT/ha (dw) could be produced annually in a large-scale wetland.
Knotgrass, on the other hand, produced less than 2 tons/acre annually in our recycling system following the fish production component. Even though the productivity of knotgrass is lower than most aquatic plants, it offers a great deal of flexibility to production systems where the quantity of flowing water varies considerably. Knotgrass in our system have been subjected to periods of no water for up to six months. During that dry period, the plant goes into a state of dormancy and then when water is added back to the system, the plant continues to grow. This type of flexible growth makes knotgrass an excellent plant for the variable flow systems. The plant is best used as cattle feed having nutritional value similar to alfalfa.
In the Texas High Plains where about five million head of cattle are produced annually in feedlots averaging about 40,000 head in size, approximately 3.6 million tons of dry biomass is produced annually. Converting this biomass to electricity via a gasifier would produce approximately 400 MWe of power, valued at about $175 million annually.
In the United States alone, if all of the livestock waste produced was recycled by applying it to land for the purpose of growing a crop and the nitrogen and phosphorous were to be applied at average agronomic rates, the land required is approximately 30 percent of the total available farmland. Not all of the livestock waste can be recycled on the land because the hauling distance often prevents this recycling method from being cost effective. Now consider the future need. Current projections indicate that our population is expected to double within the next 30 to 50 years. If we expect to maintain the same quality of life we enjoy today, over 60 percent of the available farmland will be required for merely handling the waste generated by our livestock industry. And, this has yet to factor in the need for land used to recycle municipal and other sources of wastewater.
If only half of the water from our municipal wastewater treatment systems in the U.S. were utilized to irrigate crops, over four million acres of crops could be produced. In addition, sufficient freshwater would be saved to permit our population to grow by 40 percent without adding any strain on our currently available water resources. Similarly, on a global perspective, if only half of the municipal wastewater was treated and used to produce a crop, over 180 million acres of land could be irrigated. This is equivalent to supplying over two billion people with the levels of water currently used per person in the U.S.
If all of the dry manure biomass produced in the United States was converted to a gas in a fluidized bed gasification process and the gas was used to fire a turbine-generator system, over 34,000 MWe of electrical power could be produced annually. This amount of power is valued at approximately $15 billion. If all of the cattle, swine, and poultry waste was collected in the liquid form and processed to produce water hyacinth at a production level of only 15 t/ac annually, the total biomass production potential would be 4×109 t/yr, which when converted to electricity via a biogasifier-generator system would produce nearly 500,000 MWe of power. This level of production is just shy of the 590,000 MWe of power currently consumed in the United States.
Taking into consideration the facts that available natural resources are finite and that the population will continue to grow, it is appropriate to consider the mechanisms of incorporating water reuse technology in every facet of life. It is not a matter of if we will utilize reuse technology in the future; it is a matter of when we will use it to sustain economic growth and minimize environmental impact.
Clifford B. Fedler is a Professor at Texas Tech University in Lubbock and can be contacted via e-mail at Clifford.fedler@ttu.edu. He offers special thanks to the Texas State Energy Conservation Office in Austin for their support of this research.

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