April 7, 2022 | Sam Werneke
What is Biological Oxygen Demand (BOD), and Why is it Important?:
Biological Oxygen Demand (BOD) is one of the parameters monitored by municipalities for discharge by Industrial Users. BOD (mg/L) concentration is the amount of dissolved oxygen needed for the break down of organic matter in water. When used in measuring wastewater, the concentration is an indicator of how much organic material is present in the water.
High concentrations of BOD can be found in waste streams coming from many different industrial processes. Dairy, brewing, and food processing plants typically generate the highest BOD concentrated waste streams across all industries. These industries have high concentrations of BOD due to the large amounts of organic waste, food particles, and fats/grease in the wastewater.
High BOD waste streams present problems to wastewater treatment plants and the surrounding local environments. Local wastewater treatment plants struggle to handle large amounts of BOD due to the sensitive bacteria ecosystem inside the plants. A highly concentrated BOD waste stream entering the treatment plant can throw off the health of the bacteria, which can create an environment where the bacteria die off, since all the oxygen is consumed. When an effluent with high BOD reaches surface water, oxygen levels are depleted which causes oxygen consuming organisms to die off. This dramatic shift in oxygen changes the delicate ecosystem significantly. This impact is why governments and the EPA closely monitor BOD levels discharged from industrial users and treatment plants.
Available Treatment Technologies:
Due to the damaging effects of BOD, many municipalities require industrial users to pretreat their discharge. When it comes to lowering BOD in wastewater, there are a few options that can impact it. Implementing efficient cleaning practices to lower the amount of organic material discharge from industrial processes and minimizing product waste can both help to lower the concentration of BOD. For post-production minimization, common industrial pretreatment processes, such as dissolved air floatation (DAF) systems, grease interceptors, screens, and pH adjustment, will help reduce the BOD in wastewater. Removing total suspended solids, fats, oils, and grease all lower the concentration of BOD but high concentrations of BOD in wastewater require additional treatment before discharge.
Below are two BOD removal paths for large and small volumes of high BOD wastewater.
Biotreatment involves supplying air and food, while flowing the wastewater through media with BOD eating bacteria living on it. This interaction is typically accomplished within a bioreactor with a fixed bed (FBBR), moving bed (MBBR), or membrane (MBR).
Fixed Bed Bioreactor (FBBR)
FBBR are bioreactors with a packed bed, where the wastewater flows through. The packed bed consists of a media that allows BOD eating bacteria (biofilm) to grow and allows for the wastewater to pass through. Fixed bed reactors are typically designed in series or as multiple chambers, which allows FBBRs to be effective. FBBRs typically achieve 90% reduction in BOD, with the only downside being clogging issues.
Moving Bed Bioreactor (MBBR)
Unlike FBBRs, moving bed bioreactors use suspended media to grow BOD eating bacteria, creating a biofilm. Typically, blowers keep the media suspended to interact with the wastewater and remove BOD. MBBRs eliminate the clogging issues that exist with FBBRs, but the single pass efficiency is low. Therefore requiring the wastewater to be recycled through the MBBR multiple times.
Membrane Bioreactor (MBR)
Membrane bioreactors combine filtration technology and conventional activated sludge technologies to remove BOD from wastewater. Wastewater is finely filtered through a membrane with activated sludge, removing organic solids and lowering the BOD concentration. Clogging is a common issue for MBRs, and with high concentrations of BOD or TSS, the sensitive membranes can easily be damaged.
Overall, biotreatment technologies require a large amount of space and time, have high initial costs, and create an additional sludge stream that needs to be disposed of. Biotreatment is the most cost-effective option for large volumes of high BOD wastewater.
2) Third Party Disposal
For low volumes of high BOD wastewater, third party disposal is an alternative . There are two disposal options for high BOD wastewater: private sanitation plants and organic material recyclers.
Private Sanitation Plant
Private sanitation plants are a viable option for getting rid of high BOD wastewater, as they can accept the wastewater all year round and treat it at their own plant. The issues with this process are the costs associated with hauling away the wastewater and it requires large volumes of onsite storage.
Organic Material Recycling
Organic material recycling allows for the wastewater to be recycled and used for another purpose. Most commonly the high BOD wastewater can be stored in evaporation ponds or lagoons, before being applied to the land. Using land application, the wastewater can transfer nutrients to the soil that would aid in farming. This option has the same limitations of private sanitation plants; while also being limited by location and available land. Third party disposal is a great option for treating high BOD wastewater but is very dependent on the process that generated the wastewater as it will limit whether it can be accepted.
As stated before, high concentrations of BOD are harmful to local surface water environments and wastewater treatment plants. Therefore, removing high concentrations of BOD from wastewater is an important and necessary process for many Industrial Users. Please contact Mike Hessling if you have any questions or need assistance with your high BOD waste stream.To find out more about this topic, click here.