Septage Reuse as Class A Biosolids – A Circular Bioeconomy

Abstract

In the United States, the public uses 40 billion gallons of water daily (Moupin, 2015). Much of the water that is used domestically will remain in the centralized public water system and will require treatment at a wastewater treatment plant (WWTP). However, 20% of the US does not return the water directly to the centralized water system, instead utilizing onsite septic systems to treat and disperse their water (Septic Systems Fact Sheet, 2008). Disposal methods for septage, the waste pumped from the septic tanks, are more limited than wastewater disposal methods. Wastewater treatment plants often refuse septage haulers due to the variable and unknown nature of the septage contents. The septage accepted at WWTP faces the same disposal statistics as wastewater. Approximately half (45%) of wastewater is landfilled or incinerated and the other half (55%) is land applied (Shaddel et al., 2019). Each of these disposal methods poses a risk to the environment. Landfilling the septage decreases the life expectancy of the landfill site and increases the risk of polluting the soil and nearby water supplies. Incineration releases harmful gases and chemicals into the atmosphere, and land application increases the risk to human health via the movement of contamination and pollution.This study aimed to determine if treated septage is beneficial to crop growth and soil health when used as a fertilizer and irrigation source. Durum wheat was the crop that was chosen for this study. This crop was selected due to its dual-purpose uses. The seeds can be collected for seed production, and the rest of the crop can be used for fodder. Septage dispersal and treatment are regulated by the United States Environmental Protection Agency (EPA) under the same guidelines as sewage sludge and is referred to as biosolids when treated to reduce pathogen load. For this study, the septage was dewatered using a spiral filter press and then treated using a low-temperature dehumidification system to dry it to 90% total solids or higher to produce Class A septage that are then land applied. The dehumidification system operates at a temperature and time sufficient to result in Class A biosolids according to US Part 503 EPA Alternative 1 (EPA, 2018). The thermal treatment addresses the health concerns of direct land application. To reduce pathogen load, this experiment consisted of two parallel trials, each with eight treatments. Each trial included two different application rates of the Class A septage, one application of traditional chemical fertilizer, and a control that received no fertilizer treatment. These four treatments were the same across both trials. Trial 1 received canal water flood irrigation. The other half, trial 2, were flood irrigated with the filtrate removed during the dewatering stage that was aerated and treated using a bacterial blend specific to SludgeHammer, a water treatment company, and mixed with the condensate water collected during the thermal treatment process (SludgeHammer, 2023). The findings of the study show that Class A septage can be beneficially used as fertilizer to enhance plant growth and soil health. The application of the treated filtrate water, however, was found to improve plant growth but detrimentally impacted soil health. Due to elevated pathogen levels found in the soil after the experiment, using filtrate for irrigation is not recommended without further treatment. Further research is necessary to determine the optimal application rate of Class A septage and to assess the long-term effects of using septage-based fertilizers.