Proceedings

A real-time solids odour monitoring system provides an odour management tool for DC’s Water and Sewer Authority’s (DC Water) process control and biosolids management program. The odour measurement can be related to process changes within the WWTP. Since higher odour levels means higher potential for nuisance odours at land application sites, identifying the process that contributes to these elevated levels is critical to responsible and efficient biosolids land application programs. Each year, DC Water’s 1.4 x 106 m3/d plant in Washington DC applies biosolids to over 81 km2 of agricultural land. Nuisance odours from recycling biosolids on land may drift into surrounding neighborhoods and result in neighboring communities enacting legislation to ban land application, therefore, the reduction of odour emissions from biosolids recycled on field sites is a major concern. Odours levels generated by limed biosolids are measured by headspace monitoring devices in enclosed conveyance systems. Both total reduced sulfur compounds (TRS) and nitrogen containing compounds are measured with online electro-chemical sensors. This study uses ordinary least squares (OLS) estimation and instrumental variable (IV) estimation via two-stage least squares (TSLS) to create statistical model for explaining the sources of TRS and the sources of nitrogen-containing compounds in dewatered solids and biosolids. Data analyses suggest that waste-activated percent solids (WAS %S) and dissolved-air flotation total solids (DAF TS) can contribute to mitigating TRS. However, all process variables at secondary sedimentation, which are gravity thickening percent solids (GT %S), gravity total solids (GT TS), and blend ratio, can contribute to increase TRS. The IV estimation indicates that % lime feeding, # centrifuges, cake percent solids (Cake %S), temperature at secondary effluent, and ambient temperature cannot directly explain TRS post-lime, but they do explain TRS levels via post-lime temperature. Additionally, cationic polymer at the secondary and dewatering process coupled with post lime temperature can contribute to increase N-containing compounds at the lime addition process. The accumulated cationic polymer inside the sludge of secondary sedimentation can also contribute to high N-containing compounds at the downstream.

Keywords: Biosolids, total reduced sulfur, nitrogen-containing compounds, dewatered solids, and statistical mode