Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
January, 2002

Meeting POTW COD Limits

Q. Our POTW requires COD discharge to be no more than 235 mg/l. Recently our treatment room is having hard time to meet this everyday. Our analysis of the cleaned waste water ranges from 20mg/l to 600 mg/l. We batch treat the wastewater and then dewater the sludge through filterpress. Any suggestions?

A. My usual approach is to first make very dilute samples of synthetic rinse water from raw materials, such as cleaning baths, diluted to normal use strength that eventually end up in the waste water to determine how much of an impact each waste water source will have on the waste water treatment system. You will also need to know or estimate the dragout rate which can vary with part configuration, rack design, individual operator habits, and bath condition (surface tension and viscosity can change with bath age and have a large effect on the drag out rate). Then run COD tests on the synthetic rinse samples. You may also want to grab individual rinse tank grab samples from time to time and run COD tests on them as well.

I usually find a major source of COD using this method that dwarfs the other COD sources that then helps me design a custom answer to the isolated - real problem. In some cases a low COD cleaner could be the answer unless it is the soil on the parts that is causing the high COD. I have personally seen situations where clean, neutral pH, waste water with a little phosphate, sulfate, nitrogen combined with a little sunlight led to algae blooms that went right through a filter press. After sitting for 1 to 2 days the CODs would quadruple, going from 200 mg/l to 800 mg/l! The high COD was being caused by algae in this case.

If you could tell me a little more about the process chemistry & processes producing the waste water as well as the type of waste water treatment system you are using before the filter press I could make further suggestions on this topic through the STERC ask the expert site. If you need more direct assistance I also work as a contract consultant for industry on solving this kind of problem if you reach the point where you think you may need a consultant feel free to give me a call for more details.

I hope the above comments helps you figure out where to start next. Good luck.