Just a few years ago, the decentralized wastewater field was lamenting the lack of a cost-effective method to remove nitrogen as effectively at smaller individual and clustered treatment systems as could be achieved at larger facilities. But today, small systems can match or exceed the nitrogen removal performance of the larger facilities and do it with less operation and maintenance cost.
Recalling the basic tenets of the distributed wastewater treatment approach, it follows that these technologies will feature low maintenance, favor soil dispersal or water reuse, and avoid the use of unnecessary electromechanical devices.
Onsite Nitrogen Behavior
Nitrogen is the primary element of concern along the coastal areas of the U.S. and in a few other nitrogen-
sensitive zones. Nitrogen discharges are a concern for both nitrate contamination of drinking water wells and as an aquatic plant nutrient, particularly in nitrogen-sensitive surface waters and near-shore marine waters.
Nitrogen is not readily or consistently removed in individual and cluster systems, because soil has a limited capacity to retain or remove nitrogen. Organic nitrogen generally is converted to ammonium nitrogen in the septic tank. Ammonium nitrogen is quickly nitrified as the wastewater infiltrates the aerobic soil, and nitrate–nitrogen is stable, soluble, and highly mobile in the subsurface environment. Biological denitrification of the nitrate is limited, because the subsoil is often at least somewhat aerobic and usually has very little organic carbon, which is required by the heterotrophic denitrifying microorganisms.
Therefore, where nitrogen removal is required for dispersal, pretreatment that achieves both nitrification and denitrification usually is necessary before the wastewater is discharged to the soil.
Many reasonably priced “natural” and mechanical pretreatment systems specifically designed for individual and cluster systems are available today.
The most popular example of such systems is the recirculating media filter, with timed dosing and drip or pumped dispersal. The filter media can be sand, gravel, textile, peat, or other materials. These systems are able to consistently remove 50% to 70% of the total nitrogen (TN) in the effluent, reducing it to 15 to 20 mg/L.
To achieve TN levels of 3 to 5 mg/L and lower — something increasingly being required — a denitrifying component is usually installed after the nitrification stage in the pretreatment system. To sustain a denitrification process, a reactive carbon source must be added. Commonly, methanol, acetic acid, molasses, or similar organic chemicals are used. However, the costs of constructing a feeding system and the cost of chemicals, power for a feed pump, controls, and chemical storage increase removal costs substantially.
Proprietary denitrifying units that avoid the need for feed pumps, controls, and chemicals include a slowly degradable organic material in the reactor tank that may last several years. Field results to date suggest that these system can obtain TN effluent concentrations of 3 to 5 mg/L and even lower. Since the limits of large-scale denitrification systems commonly are quoted to be 3 to 5 mg/L, these onsite systems enable similar or better effluent TN concentrations in decentralized systems, plus provide for further TN removal by the soil-pressure dosing system.
Permeable Reactive Barriers
A potentially powerful variation on these wastewater treatment systems is the permeable reactive barrier system, which consists of a trench filled with degradable carbon media. Trenches are strategically located parallel to the shoreline of a sensitive waterbody to intercept high-nitrate effluent plumes from upgradient areas. In cases where receiving waters already are eutrophied and exhibiting excessive algal blooms and die-off of the bottom grasses needed by young fish, these trenches can provide immediate relief by removing nitrate from the incoming groundwater.
If wastewater remediation is limited to reducing nitrogen at the source by installing new treatment systems, the improvements to the receiving waters directly receiving the groundwater may take 10 years or longer, given typical groundwater flows.
While nitrogen is often the main nutrient requiring control, phosphorus is a concern in areas where large numbers or volumes of wastewater discharges occur near phosphorus-sensitive surface waters. Although these are generally freshwater bodies, some marine waters (mostly along the southern coast) are equally phosphorus-sensitive.
Fortunately, in most soil-based systems, phosphorus is readily removed by chemical adsorption to soil particles and precipitation reactions with soil minerals, including iron, calcium, and aluminum. A soil’s capacity to remove phosphorus is significant both spatially and temporally. Saturation fronts of phosphorus move only inches or less per year through all but sandy or gravelly soils.
Designers can maximize phosphorus removal rates by locating the infiltration system in medium- to fine-textured soils that are as far from the surface water as possible and extending the infiltration system along the contour of the installation site.
If native soils are not amenable to adsorption removal, other adsorption methods also are available. Although some phosphorus can be removed by pretreatment systems that contain high concentrations of adsorptive elements or by biological phosphorus removal, soil adsorption is by far the most common — and least expensive — means of removal.
Clustered treatment systems offer an attractive option for nutrient removal in both new and existing developments due to economy-of-scale efficiencies and the opportunity for better system management. Regardless of the approach, comprehensive wastewater planning that includes wastewater sources, the receiving environment, and other factors (such as stormwater management facilities) can help ensure appropriate treatment at acceptable long-term costs.
is a senior environmental engineer, and
is director of applied research in the Fairfax, Va., office of Tetra Tech Inc. (Pasadena, Calif.).
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