A Review of Alternative On-Site Wastewater Technology

In last month's Lake and Watershed News, we discussed soil suitability mapping. The end result of the suitability mapping study is a set of maps depicting specific areas where either conventional septic fields or alternative on-site wastewater systems can be used. In some areas, decentralized wastewater systems are most appropriate. This month's feature article provides an overview of alternative on-site wastewater technologies.

As with any conventional septic system installation, soil testing is critical to determining the practicality of installing an alternative wastewater system. Depending on the site conditions, alternative designs such as elevated sand mounds, infiltration chamber systems, drip irrigation systems, low-pressure pipe systems, spray irrigation systems, or constructed wastewater treatment wetland systems may be used. In addition, several technologies exist that may help accelerate the wastewater treatment process and result in better pretreatment, including septic tank effluent filters and aerobic treatment units. Refer to state and local codes for specific information about wastewater system design criteria.

Elevated Sand Mounds

Raised (elevated) sand mounds may be installed where soil depths are insufficient for a conventional on-lot septic system, or where conventional septic leach fields have failed and are in need of replacement. Elevated sand mound systems consist of distribution pipes in gravel trenches situated within a mound of sand and topsoil that is placed on top of the original soil, as shown in Figure 1. A properly designed and maintained sand mound has a design life of over 20 years; therefore, raised sand mounds are considered an adequate long-term wastewater disposal method.

Infiltration Chamber Distribution Systems

Infiltration Chamber Distribution Systems are similar to conventional systems in that pretreated effluent flows from a septic tank into the leach field and eventually percolates into the ground. The difference is that instead of perforated pipe and gravel, the leach field consists of specialized chambers that are designed to increase the surface area for wastewater treatment. Therefore, these systems typically afford greater design flexibility due to the smaller footprint required (up to a 50 percent smaller leach field area). A number of U.S. manufacturers and suppliers of proprietary leaching chamber distribution systems exist. In New Jersey, Infiltrator® Chamber Distribution Systems are approved for general use with certain conditions. The infiltration chambers are typically placed over either native soil or specified fill soil in the disposal trench(es) as shown in Figure 2.

Drip Irrigation Systems

Drip irrigation systems apply treated wastewater to soil absorption fields slowly and uniformly from a network of narrow plastic, polyethylene or polyvinylchloride (pvc) tubing. The tubing is placed at shallow depths of usually six to 12 inches, in the plant root zone. The wastewater is pumped through the drip lines under pressure but drips slowly from a series of evenly spaced openings called "emitters." Wastewater must be pretreated and filtered prior to subsurface drip irrigation dispersal. One advantage to these systems is the minimal site disturbance required due to the flexible tubing that can be placed around trees and shrubs. The principal difference between drip irrigation systems and conventional on-lot wastewater systems is that irrigation systems are specifically designed to allow the water and nutrients to be used by plants.

Low Pressure Pipe Systems

Low pressure pipe (LPP) wastewater systems use one to two inch diameter plastic pipes with orifices (small spray holes) spaced 2.5 to 7.5 feet apart to deliver wastewater to the soil. A pump delivers effluent throughout the system on a regular basis as determined by a timer or the pump tank capacity. With this technology, absorption fields can be located upslope of the septic tank, or on uneven terrain that would otherwise be unsuitable for gravity flow systems.

Spray Irrigation Systems

Spray irrigation involves pumping pretreated effluent at 80 to 100 psi through nozzles and spraying it directly onto the land. Spray irrigation for wastewater disposal requires the availability of a relatively large parcel of suitable land for wastewater storage (lagoons) and disposal (spray field). However, spray irrigation systems require less usable soil depth beneath the spray field than conventional systems. Properly designed, maintained, and operated spray irrigation systems provide highly effective treatment of residential wastewater and allow for maximum recharge of groundwater, a benefit that is completely lost by small package and large community wastewater treatment facilities that use stream discharge for treated effluent. Many public facilities such as golf courses and parks are finding the beneficial reuse of treated wastewater to be a win-win situation: wastewater disposal, grounds fertilization, and drought relief are accomplished in one step.

Wastewater Treatment Wetlands

Constructed wastewater treatment wetlands utilize the nutrient-absorbing abilities of natural vegetation to treat wastewater. When properly constructed, wastewater wetlands are both attractive and effective. Constructed wetlands can offer an affordable solution to wastewater disposal in sites with failed conventional absorption fields, narrow or oddly-shaped lots, high water tables, and/or low soil percolation rates.

Two types of constructed wetlands exist: "free water surface" (FWS) wetlands, where wastewater runs through dense vegetation over a generally impervious soil surface, and "subsurface flow" (SSF) wetlands, in which the effluent runs beneath a coarse substrate such as a gravel bed matrix. Wastewater treatment wetlands can be less costly to construct and are usually less costly to maintain than traditional wastewater treatment systems. In addition, the wastewater treatment wetland recharges groundwater while providing habitat for wetland species.

The wetland system consists of a septic tank that discharges to the wetland bed. Vegetation that is adapted to saturated conditions is grown in the wetland bed, removing nutrients, organic matter, suspended solids, and pathogens from the effluent. The pretreated effluent from the wetland bed can then discharge to a land application system, as shown in Figure 3, or can be designed to be absorbed completely by the system.

Alternative On-Lot Pretreatment Technologies

In many areas, septic tank effluent filters and aerobic treatment units are approved for general use with certain conditions. Aerobic treatment units (ATUs) pretreat wastewater by adding air to break down organic matter, reduce pathogens, and transform nutrients. Compared to conventional septic tanks, ATUs break down organic matter more efficiently, achieve quicker decomposition of organic solids, and reduce the concentration of pathogens in the wastewater.

Various septic tank effluent filter types and designs have been extensively tested and used in the United States. Some wastewater treatment filters use peat, pea gravel, crushed glass, shredded recycled tires, or other experimental media, but sand is the best understood and the most predictable media. The peat filter pretreats septic tank effluent by filtering it through a two-foot-thick layer of sphagnum peat before sending it to the soil treatment system. Peat is partially decomposed organic material with a high water-holding capacity, large surface area, and chemical and biological properties that make it very effective in treating wastewater. Unsterilized peat is also home to a number of different microorganisms, including bacteria, fungi, and tiny plants. All of these characteristics make peat a reactive and effective filter.

Many other pretreatment technologies exist for wastewater systems. Denitrification devices such as Recirculating Sand Filters (RSF) utilize an additional treatment chamber between the septic tank and the leach field as a means of discharging cleaner effluent with a lower nitrogen concentration. Effluent from the septic tank is pressurized and sprayed on a volume of sand in the filter chamber. Microorganisms in the sand break down organic matter and convert ammonia into nitrate as the effluent filters through the filter. When the effluent reaches the underdrain, a portion of the water enters the leach field and the rest re-circulates through the septic tank, where the nitrates are converted into nitrogen gas, an inert gas that can be vented to the atmosphere.

Alternative on-lot wastewater alternatives may require special approvals. Alternative wastewater systems require maintenance just like conventional septic systems, and they tend to have a higher failure rate due to improper maintenance and installation. Composting toilets and low-flow fixtures can be used to reduce the amount of wastewater entering a septic system, but should not be used in lieu of a functioning wastewater treatment system. Where alternative on-lot wastewater systems are not applicable, decentralized wastewater systems should be considered.

For more information on alternative wastewater systems or decentralized wastewater, contact info@fxbrowne.com.