Each test cell in the study needs to receive a defined amount of wastewater each day, and that amount needs to be consistent across all 35 cells. The challenge is to create a dosing system that is both accurate and precise, at a reasonable cost. The solution is a modification on a tried-and-true dosing mechanism that is already in use at the Test Center: Dosing Barrels.
Dosing Barrel Design
The dosing barrels for this project are made of 5-gallon food-grade buckets. Piping is all 1″ PVC pipe. Each barrel has a PVC overflow weir which controls the liquid level in the barrel and is simple to calibrate. A small signal control float connects to the main PLC unit to check that the barrel is filled before the system is dosed.
The barrels are set up in banks of five (concurrent with the 5 replicates for each test cell type) and fill simultaneously. The influent control valve allows for balancing flow to each barrel to ensure that all are filled on each dosing run.
Gravity Distribution Barrels
The gravity dosing barrel uses a 1″ brass ball valve to control dosing to the test cell. Ball valves were selected over the cheaper solenoid valves due to the nature of wastewater – solenoid valves are prone to clogging or sticking open due to the larger solids in raw wastewater. Each ball valve is activated by the main PLC unit when the barrel is full.
Pressure Distribution Barrels
Pressure distribution is a bit more of a technical challenge for this project. Each test cell has to be dosed accurately and precisely, so it was determined early on that there would have to be one pump for each pressure distribution cell. A skimmer-type pump with 1/8″ clearance was selected to evacuate as much liquid from the dosing barrel as possible during each dose.
The effluent control valve allows for control of the amount of pressure sent to the distribution pipe. The goal is to maintain 2 feet of residual head in each pressure lateral.
Programmable Logic Controller (PLC)
This project utilizes an PLC unit based on the open-source Raspberry Pi architecture. The linux-based PLC allows for rapid software development at minimal cost.
The “Dosing Trailer”
No, not a movie trailer. The entire dosing system is built into a stripped-down former Red Cross trailer that served as disaster housing in the early/mid 2000’s. While not the most aesthetically pleasing, using the trailer allows for construction of the dosing system concurrent with construction of the test cells. When complete, the trailer will be moved to it’s final location in the middle of the test cell area and connected up. This setup allows for a shorter construction time than otherwise possible – we don’t have to wait until the test cells are complete to begin construction of the physical building and then dosing system. It also fulfills one of the Test Center tenets – recycle as much as possible.