Batch Versus Continuous Treatment
Oct 1, 1998 12:00 PM
Most industrial wastewater treatment processing can be classified as either batch or continuous systems. In a batch treatment system, a volume of wastewater is gathered, treated by chemical/physical/biological means, and discharged. In a continuous system, wastewater is continuously added and treated water is continuously discharged.
It is worth noting that a batch-treatment system operator does not have to discharge the water until he is fully satisfied that the batch has been treated properly. Conversely, with a continuous-treatment system, the operation is continuously discharging. The operator can take only a grab sample of the wastewater he is discharging and hope that the rest of the water has the same quality as the sample.
With a properly conscientious batch-treatment operator, there is no reason to discharge out-of-spec wastewater. Indeed, I have designed numerous batch wastewater treatment systems and have noted their ability to produce less out-of-spec wastewater than comparable continuous-flow wastewater treatment systems. In the real world with real wastewater treatment operators, these batch treatment systems have met with extremely high user satisfaction.
Furthermore, batch treatment systems more easily handle incoming wastewater that produces variable sludge volumes than do continuous-treatment systems. This is because the continuous system requires the operator to estimate the sludge production prior to operation in order to automate the system.
Underestimating sludge production can result in poor effluent quality from the clarifier. Overestimating sludge production can bog down dewatering of sludge. Sludge production is almost always estimated on the high side, making sludge dewatering inefficient on wastewaters that produce highly variable amounts of sludge.
The qualities that make batch treatment superior to continuous treatment also make it superior for wastewaters that call for a variable quantity of treatment chemicals. In this case, a batch system will be more efficient because the operator does not have to make conservative estimates of the quantity of treatment chemicals required.
The operator can add a moderate amount of chemicals at first. If this is inadequate, he can add more until the desired results are achieved. I have estimated that consumption of coagulants is cut by 50% when wastewater treatment operators switch from continuous to batch treatment. Reducing chemical consumption of coagulants also pays dividends in reduced sludge production which, in turn, reduces disposal costs.
Tank washes that have replaced vacuum rotary drum filters have experienced as fast as a 12-month payback. Savings are derived from the elimination of diatomaceous earth and a large reduction in the volume of sludge disposed.
Nevertheless, there has been a lot of confusion in the tank wash industry about the application of the filter press in wastewater treatment. The filter press cannot directly replace the rotary vacuum drum because it filters much too slowly.
Major revisions in the way wastewater is processed must be made to successfully utilize a filter press. Most importantly, the suspended solids (sludge) generated from treatment of the wastewater must be thickened by some means. Secondly, the sludge must be suitably conditioned to form a solid cake while in the press.
The typical means of thickening suspended solids in wastewater are by a clarifier or by an air flotation device. A clarifier is a simple sedimentation device that takes advantage ofthe fact that suspended solids, which are typically heavier than water, will fall to the bottom of a quiescent body of water. Air flotation devices utilize tiny air bubbles that attach to the suspended solids making them lighter than the surrounding water.
In air flotation, the suspended solids are skimmed off the surface of the water with a mechanical device. In sedimentation, suspended solids are removed from the bottom of the clarifier.-Bruce Bishkin
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