BOD — Biochemical Oxygen Demand — measures the amount of dissolved oxygen microorganisms need to break down organic matter in water at a given temperature. In practical terms, it’s the most direct indicator of organic pollution strength in wastewater. High BOD means a lot of biodegradable material is present; low BOD means most of it has been treated.
Raw municipal wastewater typically arrives at a treatment plant with BOD concentrations between 150 and 300 mg/L.
Australian EPA discharge limits for treated effluent are generally 20 mg/L BOD or less — so effective plants are achieving 85–95% BOD removal across their treatment train. If your plant is falling below 85%, something in the process needs attention.
This article covers five strategies for reducing BOD across primary, secondary, and tertiary treatment — including how to calculate your current removal efficiency before deciding where to intervene.
What is Biochemical Oxygen Demand?
BOD measures how much dissolved oxygen microorganisms consume while breaking down organic matter in water over five days at 20°C — which is why it’s often called BOD5. The higher the BOD, the more organic pollution is present and the more oxygen is consumed during treatment.
This matters because oxygen depletion is the mechanism through which organic pollution harms aquatic ecosystems. When high-BOD effluent reaches a river or lake, bacteria consume the available dissolved oxygen to break it down. Fish and other aquatic life suffocate. The ecological damage is directly proportional to how much BOD enters the receiving water — which is why discharge limits exist.
BOD is also closely related to COD (Chemical Oxygen Demand), which measures all oxidisable matter, not just the biodegradable fraction. BOD is always lower than COD for the same sample.
A BOD:COD ratio above 0.5 generally indicates wastewater that responds well to biological treatment; a ratio below 0.3 may suggest significant non-biodegradable content that biological processes won’t remove effectively.
How to calculate BOD removal efficiency
Before adjusting any process, calculate where your plant currently stands:
BOD Removal Efficiency (%) = [(Influent BOD − Effluent BOD) ÷ Influent BOD] × 100
Example:
Influent BOD of 220 mg/L, effluent BOD of 18 mg/L.
(220 − 18) ÷ 220 × 100 = 91.8% removal.
Secondary treatment plants typically achieve 85–95% BOD removal. If your number falls below 85%, something may be off with your process or you could be out of permit compliance. Use this baseline before deciding which of the five strategies below to prioritise.
How to Reduce BOD in Wastewater
Reduce Total Suspended Solids (TSS)
TSS and BOD move together. Suspended solids carry organic matter, and that organic matter contributes directly to BOD measurement. If your TSS levels are high, your BOD will follow — which is why primary treatment that targets TSS first makes secondary biological treatment significantly more effective.
Particles as small as 2 microns contribute to BOD test results, so primary filtration needs to be thorough. Rotary drum screens and rod sieves are commonly used to capture fine suspended solids before the water reaches biological treatment stages.
Equipment condition matters: a broken or partially blocked screen lets solids through that the biological stage then has to handle, increasing load on a process that’s less efficient at this job than mechanical filtration is.
Chemical treatment works alongside physical filtration. Digesters with chemical additives break down solid waste rather than simply capturing it, reducing disposal volume and preventing re-suspension of solids that would otherwise increase BOD downstream.
Right-Size Your Equalisation (EQ) Tank
Flow variation is one of the most overlooked causes of poor BOD performance. Industrial and municipal wastewater flows aren’t constant — they peak in the morning, drop overnight, and vary seasonally.
When high-flow periods flood a biological treatment system faster than the microbial community can process the incoming load, BOD removal efficiency drops.
The EQ tank buffers this by holding incoming wastewater and releasing it at a controlled, consistent rate to the treatment process. An undersized EQ tank doesn’t absorb enough flow variation to protect downstream processes. An oversized one can cause long retention times that change the biological character of the wastewater before treatment.
The right tank size is calculated from the plant’s hydraulic loading rate — the volume of wastewater the plant processes per unit time — which varies depending on the source water and any organic substrates added to assist treatment.
Getting this sizing right creates a stable feed for activated sludge and other biological processes, which consistently outperform systems receiving variable loads.
Increase Aeration Rates in Activated Sludge Systems
Aerobic bacteria need dissolved oxygen to break down organic matter — and organic matter is what drives BOD. Maintain adequate dissolved oxygen in the aeration basin, and the bacterial community does the heavy lifting of BOD reduction. Let DO drop below around 2 mg/L, and treatment efficiency falls sharply.
Fine bubble diffusers, positioned at the base of the aeration basin, deliver oxygen to the microbial community in fine streams that maximise surface area and absorption. As bacteria metabolise organic matter, they convert it progressively into waste sludge, which then settles in the secondary clarifier and is removed from the system.
Two things to monitor closely here. First, aeration energy cost rises steeply when you run blowers harder than needed — dissolved oxygen monitoring systems let operators maintain the target DO range without over-aerating, which is common in plants running on fixed schedules.
Second, the bacteria responsible for BOD removal aren’t the only ones in the system: maintaining the right DO levels and SRT supports beneficial aerobic bacteria while discouraging the filamentous organisms that cause sludge bulking and poor settlement.
Well-run secondary treatment with activated sludge typically achieves 85–95% BOD removal. Consistently falling below this range despite adequate aeration usually points to one of the other factors in this list — TSS loading, EQ tank instability, or temperature effects.
Coagulation and Flocculation
Not all organic matter in wastewater is in a form that biological processes can remove efficiently. More than 30% of BOD-contributing substances can be in colloidal form — particles so small they pass through physical filtration and remain suspended in the water column. Coagulation and flocculation target exactly these particles.
Coagulation introduces a chemical coagulant — typically aluminium sulphate, ferric chloride, or ferrous sulphate — that neutralises the electrical charge keeping colloidal particles separated. Once destabilised, these particles begin to aggregate. Flocculation follows, using gentle mixing to encourage the aggregated particles to bind into larger, heavier flocs that settle quickly in the clarifier.
Low-dose chemical treatment achieves BOD/COD reduction of more than 50% in chemically enhanced treatment systems, and is particularly effective for plants receiving industrial wastewater with high colloidal organic content.
The choice of coagulant matters. Iron-based coagulants — ferric chloride and ferrous sulphate — generally perform better in wastewater with high suspended solids and variable organic loading, because they’re less sensitive to pH variation than aluminium sulphate.
Studies using ferric chloride 40% solution with rotary biological contactors have shown promising results for reducing BOD, COD, and TSS to below maximum admissible concentrations in treated effluent.
Monitor and Manage Temperature
Temperature has a direct, measurable effect on BOD reduction through two mechanisms. First, warmer water holds less dissolved oxygen — at 20°C, water can hold around 9.1 mg/L dissolved oxygen; at 30°C, that drops to 7.5 mg/L.
Less available oxygen means less capacity for aerobic bacterial activity, which means slower BOD reduction. Second, bacterial metabolic rates increase with temperature up to a point, then collapse — most activated sludge communities perform optimally between 20°C and 35°C.
In Australian summer conditions, open aeration basins can experience water temperatures above 30°C, which reduces DO capacity and may stress the microbial community. In cold regions or winter conditions, temperatures approaching 10°C can slow biological activity significantly, requiring longer hydraulic retention times to achieve the same BOD removal.
The practical response is continuous temperature monitoring in the aeration basin, with DO targets adjusted based on actual water temperature rather than fixed setpoints.
A plant running a fixed DO setpoint of 2 mg/L summer and winter is operating suboptimally in both seasons. Monitoring lets operators compensate for seasonal temperature shifts before they show up as deteriorating effluent quality.
Low-Maintenance Options for BOD Reduction
Activated sludge achieves excellent BOD removal but demands continuous operator attention — aeration control, sludge wasting, DO monitoring, and regular process adjustments.
For sites where operator availability is limited, or where energy and maintenance costs need to be minimised, several lower-maintenance biological treatment options achieve effective BOD reduction:
Trickling filters pass wastewater over a media bed — typically plastic modules or rock — colonised by a biofilm of bacteria. As wastewater flows through, bacteria metabolise organic matter and reduce BOD.
Trickling filters have no moving parts in the treatment media, require minimal energy compared to aeration systems, and can achieve 70–90% BOD removal in a well-designed system. They’re widely used in regional Australian treatment plants where operational simplicity is prioritised.
Constructed wetlands route wastewater through engineered beds of gravel and wetland plants, where a combination of microbial activity, plant uptake, and filtration reduces BOD over a long retention period.
They require minimal energy and very little maintenance — the main inputs are occasional media replacement and vegetation management. BOD removal typically ranges from 70–95% in subsurface flow wetland designs, making them viable for small communities, rural properties, and remote sites.
Rotating Biological Contactors (RBCs) use slowly rotating discs partially submerged in wastewater. Biofilm growing on the disc surfaces alternates between submersion (where bacteria metabolise organic matter) and air exposure (where oxygen replenishes the biofilm).
RBCs use significantly less energy than aeration blowers, require less operator attention than activated sludge, and achieve 80–95% BOD removal. They’re particularly suited to small-to-medium plants with variable flow.
Membrane bioreactors (MBRs) combine biological treatment with ultrafiltration membranes, achieving very high effluent quality with a smaller footprint than conventional activated sludge. While the membranes require periodic cleaning, MBRs generally produce more consistent BOD removal than activated sludge under variable load conditions.
Choosing the Right Approach for Your Plant
The five strategies in this article aren’t mutually exclusive — most plants run some combination of all of them. The question is which to prioritise given your current performance data.
Start with your BOD removal efficiency calculation. If you’re below 85%, identify which stage of treatment is underperforming by measuring BOD at multiple points through the plant — after primary screening, after secondary biological treatment, and at final effluent. The stage with the largest residual BOD relative to what it should be removing is where to focus first.
For plants managing high variability in influent strength — common with industrial wastewater or combined sewerage systems — EQ tank sizing and TSS control at primary stage typically deliver the most consistent improvement. For plants with stable influent but declining effluent quality, aeration system performance, temperature effects, and sludge age are the most likely culprits.
Tigernix Smart Water Asset Solution provides real-time monitoring across the full wastewater treatment process — including BOD-related parameters, dissolved oxygen, TSS, and temperature — allowing operators to identify process deviations before they reach the effluent. If you’d like to see how continuous asset monitoring supports BOD management at an operational level, we’re happy to walk through a demonstration.
Frequently Asked Questions: BOD in Wastewater Treatment
BOD stands for Biochemical Oxygen Demand. It measures how much dissolved oxygen microorganisms need to break down the organic matter in a water sample over five days at 20°C (BOD5). In wastewater treatment, BOD is the primary indicator of organic pollution strength — high BOD means high organic load; low BOD means effective treatment or low initial contamination.
The five main strategies are: reducing TSS at the primary stage to lower organic loading on biological treatment; right-sizing the EQ tank to stabilise hydraulic and organic loading; maintaining adequate aeration to support aerobic bacterial BOD removal; applying coagulation and flocculation to remove colloidal organic matter; and managing water temperature to maintain dissolved oxygen levels and bacterial activity. The right combination depends on your influent characteristics and where BOD removal is underperforming in your treatment train.
Raw municipal wastewater typically has influent BOD between 150 and 300 mg/L. Australian EPA discharge limits for treated effluent are generally 20 mg/L BOD or less. Secondary treatment plants achieving 85–95% BOD removal are considered to be performing well. Values consistently above 20 mg/L at final effluent indicate a treatment process problem requiring investigation.
BOD Removal Efficiency (%) = [(Influent BOD − Effluent BOD) ÷ Influent BOD] × 100. For example, an influent BOD of 220 mg/L and effluent BOD of 18 mg/L gives 91.8% removal — within the expected range for a well-operating secondary treatment plant.
COD (Chemical Oxygen Demand) measures all oxidisable matter in wastewater, while BOD measures only the biodegradable fraction. BOD is always lower than COD for the same sample. A BOD:COD ratio above 0.5 indicates wastewater that responds well to biological treatment. A ratio below 0.3 suggests significant non-biodegradable content that biological processes alone won’t adequately remove.
Lower-maintenance alternatives to activated sludge include trickling filters (70–90% BOD removal, minimal energy), constructed wetlands (70–95% removal, very low energy and maintenance), rotating biological contactors or RBCs (80–95% removal, less operator-intensive than activated sludge), and membrane bioreactors or MBRs (high effluent quality, consistent performance under variable load). The right choice depends on site size, operator availability, land footprint, and influent characteristics.
