How to Lower BOD and COD Biologically

Environmental audits from the Ministry of Environment and Forestry most often find the same violation: BOD and COD in the effluent exceed the discharge standards. The result: warning letters, fines, and in serious cases, temporary operational shutdown. For food and beverage producers, hospitals, palm-oil mills, and textile factories, this is not a small problem — remediation costs can run into billions of rupiah.

The good news: lowering BOD and COD is microbial biology, not magic. The right biological approach — cheap and sustainable — can drop both parameters by up to 90% with relatively modest infrastructure investment.

What BOD and COD mean — briefly

BOD (Biochemical Oxygen Demand): the amount of oxygen (mg/L) microbes need to break down the organic matter in water over a defined period (typically 5 days, written BOD₅). It measures the biodegradable organic content.

COD (Chemical Oxygen Demand): the amount of oxygen needed to chemically oxidize the organic matter (via reaction with potassium dichromate). It measures total organic content — biodegradable or not.

Practical consequence: COD is always greater than or equal to BOD. The BOD/COD ratio tells you the quality of the waste:

• BOD/COD > 0.5 → easily biodegradable (F&B, general hospital waste)
• BOD/COD = 0.3–0.5 → still biological, needs pre-treatment
• BOD/COD < 0.3 → dominated by recalcitrant compounds (textile, chemicals) — needs chemical/physical pre-treatment

Measure this ratio before choosing a method. Many WWTP projects fail because biological technology is installed for waste whose COD is dominated by non-biodegradable compounds.

Indonesian discharge standards

Regulation MOEF No. 5 of 2014 (and subsequent revisions) sets per-sector effluent standards. Some typical figures:

SectorBOD₅ (mg/L)COD (mg/L) Hospital3080 Domestic (≥100 m³/day)30100 Food industry50100 Palm oil (POME effluent)100350 Textile60150

Always check the latest regulation — per-sector figures can be revised. The point: know your effluent target before designing the system.

Why BOD/COD is high — find the source first

Before investing in new technology, audit the source:

1. Process leaks. Product residue, oil, sugar, milk, blood — things that shouldn't enter the drainage. Identifying leak points often cuts 30–50% of the organic load with no technology cost.
2. Flow variability. A WWTP is designed for an average load. A spike (shock loading) during certain production shifts can kill the microbial culture — and COD then spikes for days afterwards while the system recovers.
3. Microbial nutrient deficiency. The ideal C:N:P ratio for aerobic systems is ~100:5:1. F&B waste is often N- or P-deficient (lots of sugar, little nitrogen) — microbes can't grow effectively.
4. Extreme pH. Microbes work best at pH 6.5–8.5. Raw POME is pH 4 — neutralization first is mandatory.

Three main approaches to lower BOD/COD

Before zooming in on biology, a quick recap:

• Physical: screening, sedimentation, flotation (DAF), filtration. Removes solids and fats. Doesn't lower dissolved COD.
• Chemical: coagulation-flocculation, advanced oxidation (Fenton, ozone). Expensive, produces chemical sludge, good for recalcitrant COD.
• Biological: use microbes to consume the BOD. Cheap, sustainable, produces less sludge that is easier to manage.

For 80% of Indonesian industrial waste cases (F&B, hospitals, domestic, livestock), biology is the primary choice. The rest need a hybrid: chemical pre-treatment → biology.

Biological approaches: two main families

Aerobic — needs oxygen

Aerobic microbes oxidize organic matter into CO₂ + water + new biomass. Result: BOD/COD drops quickly, the effluent is clear and odorless.

Popular technologies:

• Activated sludge — aeration tank + secondary clarifier. The standard for medium-to-large WWTPs. BOD removal 85–95%, COD 80–90%.
• MBBR (Moving Bed Biofilm Reactor) — a tank with plastic media that grows biofilm. Smaller footprint, more resistant to shock loading.
• SBR (Sequencing Batch Reactor) — one tank, several sequenced phases. Good for fluctuating flow.
• Aerobic biofilter — microbes grow on media (gravel, plastic) with wastewater trickling from above. Simple for small scale.

Anaerobic — no oxygen

Anaerobic microbes break organic matter into biogas (CH₄ + CO₂) + mineral sludge. Slower but more suitable for high-BOD/COD waste (>2000 mg/L) due to its energy efficiency and biogas production.

Popular technologies:

• UASB (Upflow Anaerobic Sludge Blanket) — the standard for strong industrial waste. COD removal 70–85%, biogas usable as energy.
• Anaerobic digester — for POME, livestock, food waste.
• Septic tank + bioseptik — a simple version for small domestic systems; see our article on septic tanks for details.

For a deeper look at the differences between these two systems, read Aerobic vs Anaerobic Bacteria in Wastewater Treatment.

A practical strategy: combine anaerobic + aerobic

For high-COD waste (>1500 mg/L), an anaerobic → aerobic combination is far more efficient than aerobic alone:

1. Stage 1 — Anaerobic: drop COD from 5000 → 800 mg/L (84% removal). Produce biogas as an energy source.
2. Stage 2 — Aerobic: drop COD from 800 → 80 mg/L (90% removal). The effluent is ready to discharge to the standard.

Total: 98% removal, energy generated, less sludge produced. This is the industry standard for large F&B (dairy, brewery, meat) and POME.

For small facilities (Type C hospitals, small factories), a simple MBBR + clarifier is often enough.

How to verify the BOD/COD drop

Without routine measurement, a "we've reduced it" claim won't survive an audit. The minimum to do:

1. Daily effluent samples. Use a quick COD test (test kit) for internal monitoring. BOD test weekly (it takes 5 days in the lab).
2. Monthly validation at an accredited lab. Reporting to MOEF requires a KAN-accredited lab. These results are proof of compliance.
3. 6-month trend chart. Plot daily COD/BOD. A spike on a specific day? Trace to the source — there's usually an unidentified process leak.
4. Microbial culture audit. Quarterly sludge microscopy. An experienced operator can identify filamentous bulking, dispersed growth, or dead biomass from the floc's appearance.

Most common operator mistakes

• Over-dosing nutrient or bacteria. Adding bacteria continuously without measurement is just wasteful. Add only when MLSS drops below target (typically 2000–4000 mg/L for activated sludge) or after an upset.
• Skipping routine desludging. Excess sludge = dead microbes = COD doesn't drop. WAS (waste activated sludge) must be removed routinely per the calculated SRT.
• DO (dissolved oxygen) not monitored. For aerobic systems, ideal DO is 2 mg/L. <2 mg/L: nitrification dies, COD stops dropping. >3 mg/L: aerator power wasted.
• Shock loading without an equalization tank. Industrial waste often has peak hours. An equalization tank (minimum 4–8 hours retention) smooths the load.
• Switching to cheaper bacteria without testing. Bacteria sold as "BOD/COD specialist" vary wildly in quality. Always run a 30-day pilot in a side tank before rolling out.

What Emguard offers

Emguard has two field-proven bacterial lines for wastewater treatment:

• Emguard Aerobic Bacteria — a consortium of Bacillus subtilis, Bacillus licheniformis, Pseudomonas and other cultures that effectively reduce BOD/COD in aeration tanks, biofilters, MBBR, and SBR. Used in hospital WWTPs, F&B plants, and palm-oil mills.
• Emguard Anaerobic Bacteria — an acetogen + methanogen consortium for UASB, septic tanks, and anaerobic digesters. For high-COD waste and industrial septic systems.

For every order, our team does a pre-assessment (waste data, flow, existing condition) and builds a specific dosing schedule. Consult via WhatsApp below.

Summary

Lowering BOD and COD isn't about buying the most expensive technology — it's about choosing an approach that matches your waste's characteristics. Audit the source first. Measure the BOD/COD ratio. Pick biology (aerobic, anaerobic, or a combination) according to the waste's strength. Validate routinely at an accredited lab. A system that runs consistently with an operator who understands microbiology always beats an expensive system with no monitoring.