Ensuring adequate contact time is critical to successful wastewater treatment. The measure indicates how long it takes for an amount of water or wastewater to pass through a tank at a given flow, or the period of time that a specified amount of water or wastewater remains in a settling basin, coagulating or flocculating tank or rapid-mix/gentle agitation process unit.
In storage reservoirs, contact time is the period that entering water or wastewater is held before removal for use. This dwell time can last from several weeks to years, with several months being typical.
The length of adequate time depends on the treatment. Contact time of, for example, 20 days calls for a larger process unit and tank sizes than other durations.
Theoretical examples
For example, let’s say a 720,000-gallon aeration tank size is required for 20-day contact time for an activated sludge system at 25 gallons per minute (gpm). On the other hand, a flocculation tank requires 80 minutes contact time for the same 25 gpm flow, as part of a physical chemical solid separation process. As a result, it only needs a 2,000-gallon process unit tank.
In both cases, contact time is critical to successful treatment. If removing biochemical oxygen demand (BOD) in an aeration tank requires 20 days contact time and it has only 10 days contact time, this could produce an effluent with inadequate BOD removal and unacceptably high total suspended solids (TSS) concentration. In the second example, if flocculation requires 80 minutes contact time and the actual process has 40 minutes contact time, an effluent with unacceptably high TSS concentration and other unacceptably high parameter values can result.
Consequently, the required contact time for each wastewater treatment process is unique to the wastewater and the wastewater treatment requirements.
pH system adjustment
Based on the constraints bearing on an industrial customer, a static mixer was used in a pH adjustment system. Our goal was to make the system run the way it should.
Inadequate contact time available for adjusting pH became a major obstacle, along with excessive alkaline chemical usage needed to raise pH of incoming low pH wastewater and excessive amounts of acid chemical used to lower pH of incoming high pH wastewater. The effluent pH was not only unacceptably erratic, with pH violations, but the pH adjustment wastewater treatment chemical cost was also remarkably high.
After data collection, analysis and evaluation, we determined that using two adequately sized batch tanks with approximately six hours contact time for each tank would deliver consistently compliant wastewater effluent pH. Additionally, the tanks would decrease wastewater treatment pH adjustment chemical usage.
Arriving at this outcome was pain-staking. To start, we had to determine the required amount of wastewater treatment pH adjustment chemical usage. Initially, we did not know how much pH adjustment chemical to have on hand to satisfy requirements. The next major obstacle was to capture all required wastewater sources from production processes for pH adjustment.
The outcome
As we analyzed water requirements in all production processes, we were able to better understand the volume of untreated wastewater, its pH value and the chemical requirements for necessary pH adjustment. After making the necessary wastewater treatment facility changes for this industrial client, annual wastewater treatment pH adjustment chemical usage savings exceeded $100,000, and pH violations were nonexistent.
Known in the industry as "Wastewater Dan," Dan Theobold, proprietor of Environmental Services, is a professional wastewater and safety consultant/trainer. He has more than 24 years of hands-on industry experience operating many variants of wastewater treatment processing units and is anxious to share his knowledge with others.
