Part 2 of 2: Primary Clarification
After the plant load is calculated from Plant Influent data (See Part 1 of 4), wastewater next progresses through the bar screen and grit removal areas which remove large solids before primary clarification. Operators reduce flow rate to allow heavy solids, or sludge, to settle out. Monitoring sludge concentration with a suspended solids probe ensures sending only the thickest, most water-free material through the process.
Installation of a suspended solids probe into the pipe on the discharge side of the pump enables concentration monitoring. As the pump activates and the solids concentration begins to decrease, an operator-determined set point stops the pump. Monitoring suspended solids ensures:
- Less polymer use for dewatering
- Reduction of energy
- Optimization of valuable digester space
Dissolved air flotation (DAF) works well for clarification in plants with waste solids that tend to float, rather than sink. DAF takes advantage of the floating, utilizing an air dispersion tank to lift solids to the top from where they are skimmed off. In DAF processes, measuring solids on-line optimizes the water and solids separation process and provides savings with polymer feed to optimize circuits.
The next treatment step utilizes biological processes to convert suspended solids to settleable solids. Most common processes involve:
- Activated sludge
- Trickling filter
- Rotating biological contactors (RBCs)
For our purposes, this discussion focuses on activated sludge, which uses specific aerobic bacteria to digest the particular types of solids produced by an industrial plant. Survival of these bacteria relies on optimal levels of dissolved oxygen (DO). Too high a DO level wastes energy. Too low a level may kill the bacteria or at minimum cause them distress, preventing effective breakdown of solids. Optimizing DO also prevents the accumulation of filamentous bacteria that causes bulking sludge, which interferes with solids settling.
Separation of solids also takes place in the activated sludge process. To maintain the quality of bacteria in the process, some solids return to the front of the system, a process called Return Activated Sludge (RAS). An on-line suspended solids instrument maintains the optimal proportions of RAS to Waste Activated Sludge (WAS).
In these processes, different levels of oxygen in oxic and anoxic zones are used with nitrobacteria, nitrosomonas or other bacteria to convert ammonia and nitrate to nitrogen gas. Oxygen control, critical to this process, prolongs the life of the particular bacteria used.
Similar to primary clarification, secondary clarification benefits from on-line suspended solids (mixed liquor suspended solids) monitoring to prevent clarified water from being added to the solids stream. Measurement of solids also keeps plant upsets to a minimum and reduces energy consumption since pumps can be turned off if blanket levels are too low.
Instrumentation that allows effective monitoring of the process in the stages discussed includes: