Do We Have a Large Load?

Q. We have a captive metal finishing operation and are looking at a process that will have a waste stream with 10,000 ppm Chemical Oxygen Demand at an average flow rate of 4 gal/min that will be discharging into our city’s wastewater treatment facility, which has a capacity of 2.5 million gal/day. Is this a large load for such a facility? R.N.

 

A. Although you did not identify the process, it apparently contains very significant concentrations of organics, which results in your high level of Chemical Oxygen Demand or COD. COD is the measure of the amount of oxygen required to chemical oxidize or burn the organic matter in a sample. However, most publicly owned wastewater treatment systems (we now refer to them as water reclamation facilities to be more “green”) are designed to remove biochemical oxygen demand or BOD5. The BOD5 test measures the amount of oxygen that bacteria uses over a 5-day period (hence the number “5”) in order to breakdown the organic matter in the sample.

To estimate the BOD5 capacity of your city’s wastewater treatment plant and your process’s impact, we need to make several assumptions. First, we will assume that the city’s wastewater treatment plant was designed to handle “normal” domestic sewage, such as from homes and commercial businesses, at a BOD5 concentration of 250 mg/L ( or ppm). This yields the city’s wastewater treatment BOD5 capacity at: 2.5 mgd × 250 mg/L × 8.34 = 5,213 lbs/day BOD5 .

Second, we will assume that your waste stream is fairly biodegradable with a BOD5/COD ratio of 0.5, hence, your BOD5 concentration is 5,000 mg/L (or ppm).

Last, we will assume one 8-hour shift of operation. Therefore, this produces a discharge from your process of:

(4gpm × 8hrs × 60 min/hr × 5,000 mg/L × 8.34)/ 1,000,000 = 80 lbs/day BOD5.

In summary, one shift of this process wastewater covers about 1.5% of the city’s wastewater treatment plant’s BOD5 capacity. If this is a two or three shift process, it will take about 3% and 4.5% of the treatment plant’s capacity.

While this may not seem to be a lot, your city may not want to give up this capacity to one entity since your proposed waste stream is about 20 times stronger than “normal” domestic sewage. One shift of this proposed waste stream is equivalent to the amount of BOD5 from 470 people; that is a lot of homes.

Even if your city allows you to discharge this waste stream without BOD5 removal pretreatment (assuming it is okay for pH, metals, etc.), they will likely charge you a surcharge on the amount of BOD5 over what they define as “normal” domestic sewage. For such a small flow, it is highly likely that it would be least costly to just pay the surcharge rather than pretreat the wastewater to reduce the BOD5/COD. Hope you find this helpful.