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NewsroomHow to Reduce BOD in Wastewater

  • image descriptionWastewater

Biochemical oxygen demand (BOD) is a measurement of the oxygen needed to remove organic matter from wastewater. This metric is an absolutely critical one to pay attention to for municipalities, especially ones that have wastewater treatment facilities that are aging and have had difficulty meeting permit limits in the past.

In this post, we’ll explain what BOD is, how to see if your levels are too high, and what you can do to fix it.

What Is Biochemical Oxygen Demand?

Aerobic organisms in wastewater need oxygen in order to break down organic matter. Dissolved oxygen levels in wastewater are measured soon after pulling a sample, and again after a period of time (usually five days).  The difference between the levels tells us if the BOD is high or low. 

The ultimate BOD is the amount of oxygen that would be consumed over an infinite amount of time, and is generally about twice as high as the BOD5. Carbonaceous material is oxidized more quickly, while nitrogen compounds like ammonia take longer to oxidize and are often unaccounted for in the five-day test. While the BOD5 is not a perfect test, it provides useful information about the relative cleanliness of the water.

If the levels are high, it’s a sign of excessive organic pollution.

A high BOD means there’s more food available for aerobic organisms. As a result, microbes are going to consume more oxygen, which leads to oxygen depletion wherever the water gets discharged. If BOD levels get too high, dissolved oxygen levels can plummet to zero, and only anoxic or anaerobic processes take place.

If the levels are low, it’s a sign that the water is clean. 

This measurement helps wastewater treatment professionals determine if their discharged water will be harmful to life in rivers and lakes. 

What Makes BOD Get Too High? 

In order to lower your BOD levels, it’s important first to understand what makes them high. This way, you can implement proactive measures to keep them from rising in the future. 

Here are some of the things that increase BOD: 

  • Inadequate wastewater treatment processes
  • Excessive organic matter like decaying leaves and animal remains
  • Municipal sewage
  • Fats, oils, and grease (FOG)
  • Industrial sewage from food processing, chemical manufacturing, etc.
  • Agricultural runoff like fertilizer and animal waste
  • Sludge buildup in lagoons to more than about 40% of total capacity, or more than about two feet
  • Excessive algae growth and die-off
  • Warmer waters which speeds up metabolic processes of microorganisms and makes them consume oxygen more quickly
  • Seasonal turnover of lagoons that often occurs when the bottom sludge layer warms up in the spring, or after heavy rains in the summer
  • Geese on open lagoons

There are many contributing factors to your BOD measurement. Before putting together a BOD reduction plan, observe the conditions at your system, especially color and smell. Green or clear water typically indicates healthy dissolved oxygen (D.O.) levels, and black water indicates low levels. The smell of rotten eggs indicates very low levels of D.O. and anaerobic decomposition. Consider measuring BOD5 and TSS (Total Suspended Solids) in your wastewater, measuring sludge depth, and measuring D.O. levels.

The Negative Impacts of High BOD

Here are some of the side effects of having high BOD levels: 

  • Water pollution
  • Die-off of fish and other aquatic life due to low dissolved oxygen
  • Low pH and black water
  • Disruption to aquatic ecosystems and biodiversity
  • Water quality degradation due to excessive bacteria and algae blooms
  • Foul odors and complaints from nearby residents
  • Corrosion of iron and concrete in unventilated structures caused by hydrogen sulfide off-gasing, accumulating, and oxidizing to sulfuric acid. This typically occurs on the front end of wastewater treatment, in sewer manholes and pump stations.
  • Violation of state environmental regulations.

How to Reduce BOD in Effluent

If your tests show that BOD is too high, now is the time to take action. Here are some of the strategies that can help reduce BOD levels in effluent:

  • Improvement of wastewater treatment efficiency
  • Aeration
  • Industrial pre-treatment
  • Installation and proper maintenance of grease traps
  • Seasonally discharging when effluent BOD has historically been low
  • Installation of baffles in lagoons to improve treatment efficiency
  • Irrigation with effluent
  • Construction of a wetlands cell to increase evaporation and seepage, and reduce or eliminate effluent
  • Adjust the draw-off elevation to reduce sludge in effluent
  • Sludge removal
  • Removal of algae
  • Addition of chemical oxidizers like peroxides, percarbonates, or nitrates
  • Scaring away geese with propane cannons

Some of the strategies above – like going non-discharging with a wetlands cell – can be long-term, low maintenance solutions. Other strategies – like seasonal discharge or aeration – can have a lower project cost, but can require regular testing and evaluation.

Get Help Reducing High Effluent BOD5 Levels

Typically, the best way to deal with high effluent BOD5 levels is to go non-discharging, if irrigation or building a wetlands cell is an option. Alternatively, seasonally discharging when BOD5 is low (if there is enough capacity to hold back water for six to nine months out of the year) can be a simple and easy way to meet permit limits.

EBH can prepare preliminary designs that would reduce BOD5 to within permit limits, and compare the initial and long-term maintenance costs of the most feasible options. After the most cost-effective option is selected, EBH can prepare a final design.BOD reduction isn’t the only type of wastewater treatment improvements we can help municipalities like yours with either. Learn more about our wastewater engineering services here.