Performance Evaluation of Advanced Onsite Wastewater Treatment Options

Abstract

The biologically diverse Chesapeake Bay is a resource of extraordinary productivity worthy of the highest levels of protection and restoration. It is estimated that there are more than 1.3 million homes in the Bay watershed with some type of onsite or decentralized wastewater treatment system. Improperly functioning or failing wastewater treatment systems have been one of the sources of nutrient input to the Bay. Areas of special concern include sites with high water tables and systems located near water supply reservoirs or recharge zones causing environmental or public health risk.

As part of the Chesapeake 2000 watershed restoration plan, this project involved the study of nutrient reducing onsite or decentralized wastewater treatment systems. Four residential onsite wastewater treatment systems in Anne Arundel County, Maryland were identified and monitored for their ability to reduce nutrients. Three systems were monitored for nitrogen reduction and one for phosphorus reduction. All four systems had experienced failures. Older single compartment septic tanks were replaced with multi-compartment (either 2 or 3 compartments) concrete or fiberglass septic tanks. Plastic media was added in for secondary treatment to each system with blowers to oxygenate the effluent. Air lift pumps recirculated the effluent from the second or third compartment of the tanks back into the first compartment. Treated effluent from the three nitrogen-reducing systems was disposed into drainfields while the phosphorus-reducing system disposed treated effluent via a pressure distribution system into an expanded shale bed.

All four systems were monitored weekly for one year. Twenty-four hour composite samples were collected for 52 weeks. The samples collected for nitrogen reduction were analyzed for nitrate/nitrite (NO3/NO2) and total Kjeldahl nitrogen (TKN) from which total nitrogen (N) was calculated. Samples collected for phosphorus reduction were analyzed for total phosphorus (P). Results show that when the nitrogen-reducing systems were operating properly with recirculation on, end-of-pipe average total N reductions of approximately 68% were obtained with total N concentrations of less than 14.0 mg/L. The phosphorus-reducing system achieved end-of-pipe average total P reductions of 89% dropping from an average 5.55 mg/L Average Total Phosphorus (ATP) to 0.62 mg/L ATP.

Based on these findings, researchers conclude that replacing existing residential onsite wastewater treatment systems in the Chesapeake Bay with the engineered design monitored in this study could, depending on a number of variables, result in acceptable end-of-pipe levels of nitrogen and phosphorus.