In the United States, approximately 25 million existing homes use septic tanks and onsite wastewater treatment systems. Sand bioreactors, or sand filters, are one option for onsite systems. In the past three decades, research has focused on the design, performance, and improvement of sand filters. It was well recognized that temperature was one important parameter impacting the sand filter performance. However, the temperature impacts to the performance of the sand bioreactors in the previous research were either masked or distorted by other parameters in field settings.
This research isolated temperature as the only variable to (1) quantify BOD5 removal, (2) measure the transformation of nitrogen, and (3) evaluate temporal clogging patterns in sand bioreactors under a range of constant temperatures.
A pilot-scale intermittent sand bioreactor system was used to simulate wastewater treatment processes. The system was consisted of three independent and identical septic tank-sand bioreactor systems, an operation platform and a walk-in environmental control chamber. The experimental loading rate was 240 L/m2/day (5.9 gal/ft2/day) to facilitate rapid clogging.
Sand bioreactor BOD5 removal at steady state was more than 80% at the temperature range of 5 to 20°C at the experimental loading rate. Temperature showed no significant impact to sand bioreactor BOD5 removal for first three weeks after start-up. At steady state, the highest removal percentage occurred for the lowest temperature set point and the lowest removal percentage occurred for the highest temperature set point.
No evidence of nitrification occurred in sand bioreactors at and below the temperature of 15°C before clogging at the experimental loading. Complete nitrification occurred in sand bioreactors at the temperatures of 20 and 25°C within two weeks at the same loading rate. Sand bioreactor time to clogging showed a linear relationship versus temperature in the range of 5 to 20°C. The linear equation, for 240 L/m2/day BOD5 loading rate at one dose per day is: Time to clogging (day) = 3.46 * Temperature (°C). At the same loading rate, the total BOD5 accumulative loadings also showed a linear relationship versus temperatures. The linear equation is: Total BOD5 accumulative loadings (mg/L) = 422 * Temperature (°C) – 144.