Enhancing Biological Nutrient Reduction in Advanced Individual Onsite Wastewater Treatment Systems

Advanced onsite systems need to continue to improve wastewater treatment through nutrient reduction. Some advanced onsite systems have been shown to reduce nutrients significantly. However, with modifications, these systems can reduce total inorganic nitrogen (TIN) to an even greater degree, and nitrate to levels below federal drinking
water standards.

Nitrogen compounds in water are becoming a contaminant of major concern in many areas of the country and throughout the world. Onsite wastewater treatment systems, often referred to generically as “septic tanks”, are frequently accused of contributing nitrogen to surface and ground water. Standard septic tanks and drainfields have proven unable to protect ground and surface waters in some cases. “Nitrogen removal is increasingly being required when onsite systems are on or near coastal waters or over sensitive, unconfined aquifers used for drinking water.”(USEPA. 2002) When development takes place in areas that lie beyond the sewered infrastructure, onsite systems must be able to produce effluent that protects ground and surface water from pollution. The question addressed in this investigation is, “Can Advanced Onsite Wastewater Treatment Systems be enhanced to significantly reduce nitrogen and achieve Federal Drinking water standards for nitrate.”

This study builds upon previous research where two Hoot Advanced Wastewater Treatment Systems identical to the baseline system used in this study were evaluated for six months in a moderate southern climate while under an ANSI/NSF Standard 40 performance evaluation. Waco has a humid subtropical climate with hot summers and mild winters. The average annual temperature is 67 degrees Fahrenheit (F). The warmest months are July and August which average 86 degrees F and the coolest month is January which averages 45 degrees F.

A research facility was built at the Waco Metropolitan Area Regional Sewerage System (WMARSS) Treatment Plant located in Waco, Texas. The WMARSS is a 30MGPD Plant with mixed residential/industrial use approximating 85% residential 15% industrial/commercial. This area also has an active and managed pretreatment program, ensuring that industrial/commercial sources are pretreated prior to discharge into the municipal system. Three (3) onsite systems were constructed at the research site and dosed according to the NSF Standard 40 protocol . Long term influent analyses show approximately 225 mg/l BOD, 175 mg/l TSS and 35 mg/l of TKN.

In this project, three variations of a standard Hoot Aerobic Treatment System were installed. One system was set up and operated identical to the previous study to serve as a baseline to evaluate the enhancements made on the other two systems. The systems were sampled five days per week and analyzed for ammonia (NH3), nitrite (NO2), and nitrate (NO3) and then summed to obtain Total Inorganic Nitrogen. (TIN) A Hach DR 2400 Spectrophotometer was used to analyze for NO2 and NO3, and a Hach SensION 2 with ammonia probe was used to obtain NH3.

TIN instead of TN was analyzed for several reasons. A primary consideration was expense. Purchasing the equipment and performing the analyses in house allowed us to sample 5 days per week for nitrate, nitrite and ammonia resulting in almost 250 complete sampling days in a year long study compared to only once weekly samples for the same research dollars if a commercial lab was used. The research workers in the laboratory determined slope daily, calibrated equipment and maintained the quality of samples and analyses. Split samples and spiked samples were occasionally analyzed for QA/QC with excellent results.

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