Biochemical Oxygen Demand (BOD) is a key indicator in wastewater management. It quantifies the dissolved oxygen consumed by microorganisms as they break down organic matter in a water sample. High BOD levels in wastewater signify a substantial presence of biodegradable organic pollutants. When untreated wastewater with elevated BOD enters natural aquatic environments, decomposing microorganisms rapidly deplete available dissolved oxygen, harming aquatic life and disrupting ecosystems. Reducing BOD in wastewater before discharge is a primary treatment objective.
Physical Treatment Methods for BOD Reduction
Physical methods remove larger debris and suspended solids in the initial stages of wastewater treatment. Preliminary treatment begins with screening, filtering out large items like rags and plastics to protect downstream equipment. Following this, grit removal systems, often in grit chambers, allow heavier inorganic particles like sand and gravel to settle by gravity. These processes prevent clogging and abrasion in subsequent treatment stages.
Primary treatment involves large sedimentation tanks, also known as clarifiers. Here, wastewater flow is slowed, allowing suspended organic and inorganic solids to settle to the bottom by gravity. Lighter materials like oils and greases float to the surface, forming scum that is skimmed off. Primary sedimentation can reduce suspended solids by approximately 50% to 70% and contribute to a BOD reduction of around 25% to 40%. This removal of settleable and floatable organic matter reduces the overall organic load, preparing the wastewater for biological treatment.
Biological Treatment Methods for BOD Reduction
Biological treatment is the core process for significantly reducing BOD by harnessing the natural metabolic activity of microorganisms. These microorganisms, primarily bacteria, consume dissolved and colloidal organic matter, converting it into carbon dioxide, water, and new microbial cells. This process requires a controlled environment with aeration to supply oxygen for aerobic bacteria.
The activated sludge process mixes wastewater with a suspension of microorganisms (activated sludge) in an aeration tank. Air is continuously supplied to maintain aerobic conditions, allowing the microorganisms to efficiently break down organic pollutants. After a period of aeration, the mixture flows into a secondary clarifier where the activated sludge settles, and the treated water overflows. This process can achieve substantial BOD reductions, often up to 95%.
Trickling filters use a fixed bed of media such as rocks or plastic. Wastewater is distributed over this media, forming a thin film that allows microorganisms to grow and create a biofilm. As wastewater trickles through the media, the microorganisms in the biofilm absorb and metabolize the organic matter. Trickling filters can achieve BOD removal efficiencies ranging from 69% to 90%. Rotating Biological Contactors (RBCs) operate on a similar principle, using large, rotating discs partially submerged in wastewater that develop a biological film for organic matter degradation.
Advanced Treatment and Polishing for BOD Reduction
Following physical and biological treatment, advanced methods are employed when stringent discharge limits are necessary or when water is intended for reuse. These processes further reduce remaining organic compounds and suspended solids. Advanced filtration techniques include sand filtration, which removes fine suspended particles, and membrane filtration. Microfiltration and ultrafiltration membranes, with sub-micron pore sizes, can remove nearly all suspended solids and insoluble BOD, and some larger soluble BOD molecules. Membrane bioreactors (MBRs) combine biological treatment with membrane filtration, offering very high BOD removal, often up to 99%.
Chemical oxidation processes use strong oxidizing agents like ozone or hydrogen peroxide to break down recalcitrant organic compounds that are not easily biodegradable. These agents convert complex pollutants into simpler, less harmful substances. Adsorption, typically using activated carbon, is another advanced method. Activated carbon has a porous structure that attracts and traps dissolved organic pollutants on its surface, effectively removing them from the treated water. These polishing steps ensure the effluent meets stringent quality standards for environmental discharge or water reclamation.
Monitoring and Maintaining Low BOD Levels
Consistent monitoring of BOD levels in treated wastewater is essential to confirm the effectiveness of treatment processes and ensure compliance with environmental regulations. The standard method for this assessment is the 5-day Biochemical Oxygen Demand test, often referred to as BOD5. This test measures the amount of oxygen consumed by microorganisms over a five-day period at 20°C, providing a reliable indication of the remaining biodegradable organic matter in the water. A low BOD5 value in the final effluent indicates successful removal of organic pollutants.
Maintaining consistently low BOD levels requires ongoing operational adjustments and meticulous maintenance practices within the treatment plant. For instance, in biological treatment systems, optimizing aeration rates ensures microorganisms receive adequate oxygen for efficient organic breakdown. Effective sludge management, including the return of active biomass to aeration tanks and the removal of excess sludge, is also crucial for sustaining microbial activity. Regular equipment inspections and cleaning protocols contribute to the overall efficiency and reliability of the entire treatment system, preventing operational issues that could lead to increased BOD in the discharged water.