Evaluation of Equipment for the Screening of Digested Solids Prior to Centrifugation: A Case Study at the Joint Water Pollution Control Plant

In order to protect the dewatering centrifuges and to enhance the marketability of the dewatered biosolids, the digested solids stream is screened prior to centrifugation at the Joint Water Pollution Control Plant in Carson, California. This facility is owned and operated by the County Sanitation Districts of Los Angeles County. Five different screening machines were tested. Four were septage acceptance units and one was an in-line strainer. The candidate units would replace a set of nine rotary drum screens currently in use, as well as two screenings dewatering presses. The in-line strainer (complete with pneumatic back pressure control, screen and auger), as well as two septage acceptance units (complete with screens and augers only) were found to be acceptable for this application. The in-line strainer was more expensive, but could remove as much as 997 wet lb/Mgal (119 g/m3) coarse screenings at a higher total solids (TS) content (52.2%) than either of the acceptable septage receiving units, which could remove 661–782 wet lb/Mgal (79.1–93.6 g/m3) at 24–28% TS. The in-line strainer removed nearly twice as much material (by dry weight) as the current screens which remove 1,000 wet lb/Mgal (120 wet g/m3) at 24–25% TS. This report describes the operational testing program and the method of evaluation for the five machines which were tested.