Knowledge Base

Resources & FAQ

Learn wastewater treatment fundamentals, find answers to common questions, and browse our glossary of industry terms.

What is the difference between BOD and COD?

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BOD (Biochemical Oxygen Demand) measures oxygen used by microorganisms to decompose organic matter over 5 days. It indicates biodegradable organic pollution.

COD (Chemical Oxygen Demand) measures oxygen equivalent of all organic matter that can be oxidized chemically. It includes biodegradable and non-biodegradable organics.

The BOD/COD ratio indicates biodegradability. Ratios >0.5 suggest readily biodegradable wastewater suitable for biological treatment.

How does temperature affect biological treatment?

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Biological reaction rates increase with temperature. Nitrification is particularly sensitive: rate halves at 10°C compared to 20°C. Design must account for winter temperatures.

Anaerobic digestion performance decreases below optimal temperatures. Heating digester to maintain temperature is a significant energy cost.

What causes bulking sludge and how is it controlled?

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Sludge bulking is excessive growth of filamentous bacteria that interfere with settling. Indicators: high SVI (>150 mL/g), cloudy effluent.

Causes: low DO (<1 mg/L), nutrient deficiency, low F/M ratio, septic influent.

Control: maintain DO >2 mg/L; install selectors; chlorinate return sludge; improve screening.

What treatment process should I choose?

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Selection depends on: wastewater characteristics (flow, BOD, contaminants), effluent quality requirements, available space, budget, and energy costs.

Primary only: Simple pretreatment, limited removal.

Secondary (biological): Standard municipal/industrial treatment, 85-95% BOD removal.

Tertiary: Nutrient removal, filtration, disinfection for strict standards.

Advanced (MBR/RO): Water reuse, zero liquid discharge.

How often should equipment be maintained?

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Maintenance frequency varies by equipment:

Screens: Daily inspection, monthly cleaning
Blowers/aerators: Quarterly inspection, annual service
UV lamps: Annual replacement
Membranes: Quarterly cleaning, 5-10 year replacement
Pumps: Annual inspection, seal replacement as needed

What is the typical cost of a wastewater treatment system?

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Costs vary widely by capacity and treatment level:

Packaged plant (50-500 m³/day): $50,000 - $500,000
Custom secondary system: $200,000 - $2,000,000
MBR system: $300,000 - $3,000,000
ZLD system: $1,000,000 - $10,000,000+

Operating costs typically range from $0.15-1.20 per m³ depending on treatment complexity.

Activated Sludge

Biological treatment process where microorganisms in aerated tanks break down organic matter. The biomass (activated sludge) is settled and returned to the aeration tank.

BOD (Biochemical Oxygen Demand)

Measure of organic matter in water expressed as oxygen consumed by microorganisms during decomposition. BOD₅ measures oxygen demand over 5 days at 20°C.

COD (Chemical Oxygen Demand)

Measure of oxygen equivalent to organic matter that can be oxidized chemically. COD is typically higher than BOD and provides rapid assessment of pollutant load.

Clarifier

Tank where suspended solids settle out by gravity. Primary clarifiers remove settleable solids; secondary clarifiers separate biomass from treated water.

HRT (Hydraulic Retention Time)

Average time wastewater remains in a treatment reactor. Calculated as reactor volume divided by flow rate. Key design parameter for biological processes.

MBR (Membrane Bioreactor)

Combined activated sludge and membrane filtration system producing high-quality effluent. Membranes (UF) replace secondary clarifier for solids-liquid separation.

MLSS (Mixed Liquor Suspended Solids)

Concentration of biomass and suspended solids in aeration tank, measured in mg/L. Typical range 2,000-4,000 mg/L for conventional activated sludge.

SRT (Solids Retention Time)

Average time biomass spends in system. Longer SRT provides better treatment but produces less sludge. Controls nitrification capability.

F/M Ratio

Food-to-Microorganism ratio measuring organic loading relative to biomass. Typical range 0.2-0.4 kg BOD/kg MLSS-day. Controls process performance.

TSS (Total Suspended Solids)

Solids that can be filtered from water, including organic matter and inorganic particles. Key water quality indicator.

UV Disinfection

Disinfection using ultraviolet light at 254nm wavelength to damage microbial DNA. Chemical-free but requires adequate UV transmittance and lamp maintenance.

ZLD (Zero Liquid Discharge)

Treatment approach eliminating liquid effluent discharge. Uses evaporation, crystallization, and membrane processes to recover all water.

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