Land Application for Natural Wastewater Treatment
The U.S. Environmental Protection Agency (EPA) and individual states regulate wastewater discharge to surface waters through the National Pollution Discharge Elimination System (NPDES) program. Because water quality standards are high and effluent limits restrictive, process water and wastewater can be expensive to treat. Due to the large volumes of water requiring treatment, many industries and municipalities are using land application to avoid discharge to surface water. Land application is a beneficial re-use of these wastes and typically costs 30 to 50 percent less to operate than high energy demanding mechanical treatment options. In addition, recycling water just once cuts water demand in half.
Chemical constituents permitted through NPDES are contaminants in our streams and rivers, but many can become nutrients for soil when wastewater effluent is treated by land application. The 1998 Clean Water Action Plan directed the EPA to develop discharge criteria for nitrogen and phosphorus (as well as other constituents) to limit biological growth in surface water. Wastewater generators are faced with tough decisions and major investments in secondary and tertiary treatment processes to meet these criteria, and yet may still face additional restrictions on discharge characteristics, such as temperature, due to regulated total maximum daily loads (TMDLs).
In a wastewater land application treatment system, conventional treatment methods are supplemented or bypassed with the application of wastewater to agricultural fields, using the biological treatment and filtering capacity of the soil, plus crop nutrient uptake and water use to manage the waste. Agricultural crops, including forages (alfalfa hay, grass, hay), grains, vegetables and hybrid poplars often generate a secondary revenue stream providing a partial offset on treatment costs. Regulatory compliance is maintained in groundwater, NPDES permitting is reduced or eliminated and the water is recycled, particularly for the agricultural sector, which uses more than 70 percent of all fresh water. With proper management, a broad range of waste streams from power producers, food processors and municipalities can be land applied.
Temperature and phosphorus in cooling tower blow-down water from power generation are excellent candidates for natural treatment by land application. |
A New Use for Power Plants' Wastewater
Temperature and phosphorus in cooling tower blow-down water from power generation are excellent candidates for natural treatment by land application. What was once a 700,000 gallon a day wastewater dilemma is now the source of irrigation water over approximately 190 acres planted with Bermuda grass for an 820 megawatt natural gas- fired and fuel oil power plant in Texas. Management of soluble salts, important for maintaining soil productivity and fertility, made the characterization of baseline soil chemistry and hydraulics combined with development of a site-specific crop management plan critical for successful land treatment. Agronomists and soil scientists developed a cropping plan for this site to match the nutrient requirements and salt tolerance of Bermuda grass to the nutrients available in the saline wastewater from the generator's cooling towers. The resulting crop is harvested as hay and sold to nearby horse feed markets. Every crop harvest yields income, while naturally recycling the facilit
y's water.
Although wastewater land application systems must be designed to accommodate the needs of individual facility and site-specific conditions, it makes good business sense to look for wastewater solutions that are both cost-effective and environmentally sustainable. This Texas energy producer saved an estimated 10 to 30 percent over other methods of water treatment, met the Texas Natural Resource Conservation Commission permit requirements and implemented the $1.5 million project within eight months. The natural wastewater treatment approach included 15,000 feet of pipeline, a pump station, mechanical pivot irrigation, and controls.
A Substitute for Agricultural Fertilizers
Land application reduces reliance on stressed fresh water sources and can substitute for commercial fertilizers. |
Land application reduces reliance on stressed fresh water sources and can substitute for commercial fertilizers. Important to soil fertility is nitrogen, a critical nutrient for plant growth and the most common component of agricultural fertilizers as well as many wastewaters. Agricultural application of nitrogen-based fertilizers is believed to account for concentrations of nitrate-nitrogen approaching or exceeding the safe level of 10 parts per million (ppm) in many rural water supply wells across the United States. Elevated concentrations of nitrate in drinking water can cause methemoglobinemia in infants (blue baby syndrome) and miscarriages in pregnant women.
The full impact of leached fertilizer nitrogen on ground water is slow to develop in many areas, because the transport of solutes through the unsaturated soils is slow. With sources traced to agricultural fertilizers, animal feedlots and septic systems, concentrations of nitrate-nitrogen ranging between 53 and 540 ppm have been reported in California and 83 ppm in water supply wells in Arizona. Rivers and streams in agricultural areas have also exhibited nitrate concentrations exceeding 10 ppm.
Reuse of wastewater nitrogen by land application applies nitrogen in a controlled manner with small doses less susceptible to leaching and reduces the need and cost of commercial fertilizer. Nitrogen fertilizers require large amounts of energy to produce, so reducing agricultural fertilizer application makes sense, as does recycling the water.
Reusing Food Processors' Wastewater
Many food processors have already recognized that land application of their process water is a natural and economical choice for wastewater management. Irrigating wastewater back to crop land has definite economic advantages over complicated mechanical treatment systems. Fresh water used for processing food crops generates wastewater high in nutrients during washing, peeling and processing. Simple irrigation systems using food processing wastewater, coupled with supply wells to provide sufficient irrigation volume, recycles water and nutrients, including nitrogen and phosphorus out of the waste stream. Management of food processor land application sites includes operation and maintenance programs developed by agronomists, soil scientists, and hydrologists. Water balances are managed to match crop water use and salt leaching needs with irrigation to maintain water percolation to ground water within the system design. Nitrogen balances are developed to control gross nitrogen additions to match estimated
nitrogen losses and crop uptake (removal) to minimize percolate nitrate losses to groundwater.
Many food processors have already recognized that land application of their process water is a natural and economical choice for wastewater management. |
Careful monitoring and management of nitrogen at a potato processing facility in the Pacific Northwest has allowed for the reduction of pre-existing nitrate contamination in the groundwater at their land application site. Nitrogen is found in both the process water and the supplemental groundwater used for irrigation. Past inefficient agronomic management of these fields, more than 40 years ago, had resulted in impact to the groundwater. Later field development, including the installation of supplemental irrigation supply wells (most located near the center of each pivot irrigation system), has resulted in a 2,300-acre irrigated farm for process water treatment by land application.
Model simulations indicate that groundwater travel time allows for the effective capture and recycling of the pre-existing nitrate-rich plume. The irrigation and crop management plan guides crop selection, application rates and frequency of irrigation, thereby controlling percolate loss and the leaching of nitrate through the soils. Crop tissue monitoring, as well as soil and ground water monitoring, confirm that crop uptake of applied nitrogen is effectively solving a prior groundwater problem.
Many municipalities negotiate with adjacent farmers for treatment of municipal effluent by land application. An example system is the City of McCall, Idaho, where 2.5 million gallons a day (MGD) of secondary-treated effluent is mixed with fresh water for delivery under gravity flow to 4,200 acres of pasture and seed potato ground. The mechanized system includes automated flow measurement and monitoring with trans-sonic flow meters for accurate mixing of the effluent and fresh waters. In Rockingham Country, Va., five MGD of treated effluent is land applied on a 700-acre farm, with fully mechanized irrigation systems coupled with soil moisture and nutrient monitoring. The controlled application and flow of effluent through the soil profile allows for the uptake of nutrients by the crop and a reduced risk of concentrated effluent constituents discharging to the groundwater or surface water.
Economic Advantages
Natural wastewater treatment by land application is generally less capital intensive than secondary and tertiary treatment. Capital investment is 10 to 40 percent less than alternative treatment methods, and this gap should widen as the discharge standards for NPDES permits become stricter. Land application systems are also typically 30 to 50 percent less expensive to operate, generally requiring fewer people, less pumping energy, less equipment maintenance and providing crop income to offset costs. In addition, management is simplified, because land-based systems tend to be more forgiving to variations in the waste stream because of the water and nutrient holding capacity of the soils. Typical systems cost from $1 million to $20 million to construct, handle from 0.5 million to 5+ million gallons of water per day and have annual operating costs ranging from $0.1 to $2.0 million.
In the world of increasing population and the resulting increase in demand on our fresh water supply, natural treatment and reuse of wastewater by land application is a sustainable management strategy. Land application technology stretches the fresh water supply and protects the quality of surface and groundwater. What would be considered pollutants in our streams and groundwater become nutrients to crops, and constituents such as nitrate and phosphorus are recycled and managed. Natural treatment of wastewaters by land application only makes sense.
This article originally appeared in the August 2001 issue of Environmental Protection, Vol. 12, No. 8, p. 28.
This article originally appeared in the 08/01/2001 issue of Environmental Protection.