February 2007, Vol. 19, No.2
Nutrient removal at home
Nearly one-fourth of U.S. households are located beyond the reach of municipal sewers. Traditionally, wastewater treatment for these homes has been provided by soil-based onsite systems — more commonly known as septic systems — which are installed on individual properties or, in some cases, on a remote lot to treat the wastewater from several homes in a cluster system. These systems originally were designed to protect human health by discharging the wastewater into the soil below the ground surface and at a distance from any wells. Septic systems were considered to fail when wastewater ponded on the ground surface or backed up in the home plumbing. System failures usually went unnoticed by regulators unless a neighboring property owner filed a complaint. Even then, most regulators tolerated poorly performing systems because they were considered to be only interim solutions until sewers were available. This is no longer the case.
In its 1997 “Response To Congress on the Use of Onsite and Decentralized Wastewater Management Systems,” the U.S. Environmental Protection Agency (EPA) stated that onsite and cluster systems are a necessary and critical component of our wastewater infrastructure and, if properly sited, designed, operated, and managed, they are a technically and economically appropriate means for treating wastewater in many small unsewered communities. EPA’s statement placed managed onsite and cluster treatment technologies and practices on the continuum of wastewater treatment options to meet our water quality goals for all discharges, regardless of their size.
The EPA statement alone was not enough for onsite and cluster technologies to gain parity with municipal treatment technologies and practices. If water quality goals are to be achieved, onsite and cluster systems need the capabilities to meet discharge requirements determined by the sensitivity of the receiving environments in which they are installed. Although traditional onsite and cluster systems provide advanced treatment levels achieving nearly complete removal of biochemical oxygen demand, total suspended solids, fecal indicators, and parasites as the water percolates through unsaturated soil, they were not designed to remove nitrogen and phosphorus.
Designing onsite and cluster systems for nitrogen and phosphorus removal presents several unique challenges. Influent flows are intermittent and extremely variable, economies of scale do not exist or are very small, and the systems are typically privately owned and inadequately operated. On the other hand, with their small-volume discharges from scattered installations throughout a watershed and ultimate soil dispersal, onsite and cluster systems better utilize the assimilative capacity of the environment and reduce overall risks from any system malfunctions. Electromechanical systems, which support sensitive biological processes (such as activated sludge) that need regular attention and maintenance, do not perform well under these conditions. Onsite and cluster systems do perform well in these conditions; however, they require comprehensive professional management to meet more demanding performance goals.
Nitrogen discharges are a concern near drinking water wells, nitrogen-sensitive surface waters, and nearshore marine waters. Standard onsite and cluster systems do not remove nitrogen consistently, because soil has a limited capacity to retain it. Organic and ammonium nitrogen are nitrified quickly as wastewater infiltrates the soil. However, biological denitrification of the nitrate is limited because the subsoil usually has very little organic carbon, which is required by the heterotrophic denitrifying microorganisms. Therefore, where nitrogen removal is required, it usually is necessary to achieve both nitrification and denitrification during pretreatment before the wastewater can be discharged to the soil.
Today, there are many reasonably priced “natural” and mechanical pretreatment systems for onsite and cluster systems. Most designs feature either simultaneous denitrification or nitrification followed by recycling back to an anoxic reactor. Examples of such systems include recirculating media filters (sand, gravel, textile, etc.), sequencing batch reactors, extended aeration with recycle, fixed-film activated sludge with recycle, and timed dosing with drip or pressure dispersal. These systems can consistently remove 50% to 70% of the total nitrogen (TN), which reduces the TN in domestic wastes to 15 to 20 mg/L.
Near drinking water wells or where it is necessary to meet total maximum daily load goals in impaired waters, the requirements are becoming more stringent, such as achieving TN concentrations of 10 mg/L or less. In such instances, a denitrifying component is installed after a nitrification stage, to which a carbon source is added. Commonly, methanol, acetic acid, or similar chemicals are used. However, the costs of installing and maintaining the chemical feed pump, controls, and chemicals and their storage increase the costs of removal substantially. Proprietary denitrifying units that avoid the need for feed pumps, controls, and chemicals — and hence are less costly and have fewer maintenance requirements — are showing promise. These units, which consist of a slowly degradable organic material placed in a reactor tank, can remove nitrogen passively for several years. Results suggest that effluent concentrations of less than 5 mg/L are possible.
Phosphorus is a concern in areas where large numbers of wastewater discharges occur near sensitive surface waters. Fortunately, phosphorus is removed readily in most soil-based systems through chemical adsorption on soil particulates and precipitation with minerals in the soil, including iron, calcium, and aluminum. Saturation fronts of phosphorus move only inches or less per year through all but the more coarse soils. To maximize phosphorus removal, the infiltration system should be located in medium- to fine-textured soils as far from the surface water as possible and stretched along the contour of the installation site. When positioning options for these systems are inadequate, unit process options that rely on media surface chemical precipitation or adsorption can be an effective alternative.
It is possible for onsite and cluster systems to achieve consistent nutrient removal to very strict concentration limits at a reasonable cost. Overall community costs should come down even further as regulatory agencies come to understand that soils and receiving environments can remove substantial amounts of nutrients. As we are better able to predict the fate and transport of the nutrients through soils and groundwater, advanced treatment may be necessary only in areas with concentrated populations in more sensitive environments. With the increasing demand for responsible management entities to operate and maintain onsite and cluster systems, low-cost advanced treatment systems that perform more reliably and have fewer operational demands are becoming common tools that can be integrated effectively with municipal treatment facilities to achieve our water quality goals.
Richard J. Otis is a senior associate at Ayres Associates (Madison, Wis.).