Many wastewater treatment plants process municipal, industrial and septic tank sludge that contains debris and inorganic solids, such as hair, plastics, grease and hygienic materials. This debris can negatively impact downstream equipment by clogging pumps and valves, and decreasing the effectiveness of aeration, dewatering and filtering equipment.
Encountering the problem
The Wheaton Sanitary District in Wheaton, Ill., encountered these problems when septage haulers would unload into a manhole in front of the district’s screens. Stones and heavy debris clogged the screens and were forced through during high flow events, which damaged other equipment at the plant.
After years of troublesome operation, the plant performed experiments using major manufacturers’ septage receiving units. During the tests, numerous units failed to handle high loads, blinded up and didn’t allow haulers to unload efficiently. The Lakeside Raptor Septage Acceptance Plant (SAP)  came out ahead by proving it could handle the challenge of high flows, stones and large debris. As a self-contained, fully automated unit, the SAP incorporates the Raptor Fine Screen for screening, dewatering, conveying and compaction.
The SAP’s  3-plane Fine Screen is one of few on the market that are equipped with a rotating rake that passes through the full depth of the basket bars, removing debris from the screening area. The rotating rake then deposits collected screenings into a central screw conveyor hopper that leads to the transport tube. Screenings are spray washed in two stages to return organic materials to the liquid stream. The first wash is over the screen basket, while the second takes place in the transport tube just before the compaction zone, washing organics back into the flow stream. The all stainless steel shafted screw conveyor transports washed screenings to the discharge chute. When debris drops to the storage container, the total solids content is typically 40% and passes the Environmental Protection Agency paint filter test.
Finding the best solution
In 2001, the Wheaton Sanitary District installed the SAP  with dual inlet valves, hauler access systems and flowmeters, creating two separate unload stations. With dual valves, two septage haulers can unload side by side without causing any interference to the SAP’s  total capacity. Each station can accept 600 gal per minute (gpm), for a full capacity of more than 1,000 gpm with no challenge to the unit.
The automatic controls at each station feature a PIN keypad where haulers can activate the system, and a printer supplies receipts that note the time and date of each load. Flowmeters, measuring within a few gallons, ensure accurate reports for billing. Operators can then print summary reports of all loads to track the SAP’s  usage.
Many haulers in the Chicago area prefer the Wheaton unit due to its efficiency and reliability. Some brands of septage receiving stations will shut down when haulers discharge too quickly, but the SAP  has yet to experience any issues during its five years of operations. When both stations are in operation, the unit has rarely reached its capacity. Plant operators also favor the SAP  for its easy operation, hauler access system and invoicing program. The program minimizes administrative hours and unload times, while creating cost paybacks and drawing more haulers.