Understanding Dairy Effluent: Key Characteristics

Dairy processing plants generate massive volumes of wastewater every day. For every litre of milk processed, roughly 2.5 litres of wastewater are produced. This wastewater – known as dairy effluent – is far from harmless. It carries a complex mix of milk residues, fats, proteins, sugars, cleaning chemicals, and dissolved solids that can seriously damage aquatic ecosystems and soil health if released without treatment. To design effective treatment systems, engineers and dairy managers must first understand what exactly is in this wastewater. That starts with studying the physical, chemical, and biological characteristics of dairy effluent.

What is dairy effluent? 🔗

Dairy effluent is the collective wastewater generated during all stages of dairy processing – from milk reception and pasteurisation to cheese-making, butter production, and equipment cleaning. It is not simply diluted milk. It is a mixture of milk proteins, lactose, fats, cleaning agents (acids, alkalis, sanitisers), and various suspended and dissolved particles. The composition changes depending on the type of dairy products being manufactured, the scale of the facility, and the cleaning-in-place (CIP) protocols used at the plant.

For instance, a cheese-manufacturing unit typically produces effluent with a much higher pollutant load than a liquid milk pasteurisation plant. Similarly, butter and cream production generate wastewater with significantly higher fat concentrations. Seasonal variations in milk production – which often peaks in summer – further affect both the volume and strength of dairy effluent.

Physical characteristics of dairy effluent 🔗

The physical properties of dairy wastewater are the first indicators of its pollution potential. These parameters help treatment plant operators quickly assess effluent strength and decide on appropriate treatment steps.

Total solids (TS) 🔗

Total solids represent all the matter that remains after the water in a sample is evaporated. In dairy effluent, TS typically ranges from 1,000 to 4,000 mg/L. These solids include both dissolved substances (like lactose and mineral salts) and suspended particles (like coagulated proteins and fat globules). TS is a critical design parameter because it directly affects the sizing of treatment equipment and the choice of treatment technology.

Total suspended solids (TSS) 🔗

TSS refers to the particulate matter that causes visible cloudiness in wastewater. In dairy effluent, TSS values typically range from 100 to 1,000 mg/L, though values up to 1,500 mg/L are reported in some facilities. The particles mainly include coagulated milk proteins, fat droplets, and processing debris. High TSS can clog sewer pipes, interfere with biological treatment processes, and increase sludge volumes.

Total dissolved solids (TDS) 🔗

TDS represents the invisible dissolved components in dairy wastewater – primarily lactose, mineral salts, and residues of cleaning chemicals. TDS values in dairy effluent typically fall between 700 and 2,500 mg/L. Unlike suspended solids, dissolved solids pass through standard filters and require specialised treatment methods such as membrane filtration or reverse osmosis for removal.

Colour and turbidity 🔗

Dairy effluent is typically white in colour with a turbid, hazy appearance. The white to cream colour comes from casein proteins and emulsified fat. Turbidity, measured in Nephelometric Turbidity Units (NTU), generally ranges from 100 to 800 NTU – far higher than the 1-4 NTU considered acceptable for drinking water. The degree of turbidity directly reflects the concentration of suspended organic matter in the effluent and serves as a quick visual indicator of pollution strength.

Odour 🔗

Fresh dairy effluent typically carries a sour milk smell caused by lactic acid formation as lactose ferments. However, if the wastewater sits in collection tanks or pipelines without aeration, anaerobic bacteria take over and produce hydrogen sulphide (the characteristic rotten egg smell) and other foul-smelling compounds. Casein that precipitates from dairy waste can decompose into a highly odorous black sludge, making timely treatment essential for odour control.

Temperature 🔗

Dairy wastewater is typically warmer than municipal sewage, with temperatures usually ranging between 17-25 °C depending on the processes involved. Hot water used in pasteurisation and CIP cleaning raises the effluent temperature. This warmth accelerates microbial activity and fermentation, which can intensify odour problems but also makes the effluent more amenable to biological treatment.

Chemical characteristics of dairy effluent 🔗

Chemical parameters reveal the true pollution load of dairy wastewater and are the most important factors in designing treatment systems.

Biochemical oxygen demand (BOD) 🔗

BOD measures the amount of oxygen that microorganisms need to break down organic matter in wastewater over a standard five-day period (BOD₅). It is one of the most widely used indicators of organic pollution. Dairy effluent typically has BOD levels of 1,000 to 4,000 mg/L – roughly 5 to 10 times higher than domestic sewage, which averages 200-400 mg/L.

This high BOD is mainly due to the presence of lactose, milk proteins, and fats. Among dairy products, cream (with 40% fat) can have a BOD₅ of approximately 400,000 mg/L, while skim milk has around 70,000 mg/L. If discharged untreated into a river or lake, high-BOD wastewater rapidly depletes dissolved oxygen, suffocating aquatic organisms.

Chemical oxygen demand (COD) 🔗

COD measures the total amount of organic pollution that can be oxidised using a strong chemical oxidant such as potassium dichromate. COD values in dairy effluent typically range from 1,500 to 6,000 mg/L, though whey-heavy waste streams can reach much higher levels.

The COD-to-BOD ratio is an important indicator of biodegradability. For dairy effluent, this ratio generally falls between 1.5:1 and 2.5:1. According to the Dairy Processing Handbook published by Tetra Pak, the COD/BOD₅ ratio for dairy plants producing liquid milk, butter, or cheese averaged around 1.45, while plants producing milk powder, whey powder, and casein had a higher average of about 2.14. A lower ratio indicates that the organic matter is mostly biodegradable – which favours biological treatment approaches.

pH 🔗

The pH of dairy effluent fluctuates widely – typically between 2 and 12 – depending on the cleaning chemicals in use. Acid washes (phosphoric or nitric acid) push pH downward, while alkaline caustic soda washes push it upward. Milk and butter production generate effluent with a near-neutral pH of around 6.8-7.4, while cheese manufacturing with acid coagulation can produce acidic whey with a pH as low as 4.3-4.6. Biological treatment systems work best within a pH range of 6-9, so most dairy treatment plants include a pH equalisation tank before the main treatment stages.

Fats, oils, and grease (FOG) 🔗

Dairy wastewater contains significant quantities of fats, oils, and grease, mainly from milk fat that enters the waste stream during processing, spills, and equipment cleaning. FOG is one of the most persistent pollutants in dairy wastewater because milk fats are often emulsified, making them resistant to simple gravitational separation. Without dedicated pre-treatment – usually dissolved air flotation (DAF) – FOG sticks to pipe walls, causes blockages, disrupts biological treatment by forming a surface layer that restricts oxygen transfer, and leads to sludge floating problems.

Nutrients: nitrogen and phosphorus 🔗

Dairy effluent contains elevated levels of nitrogen (14-830 mg/L) and phosphorus (9-280 mg/L), primarily derived from milk proteins, lactose, and cleaning chemicals. Nitrogen in dairy wastewater exists mainly as amino groups from milk proteins, with smaller contributions from urea, ammonium ions, and nitrates. Phosphorus is present in both inorganic forms (phosphates) and organic compounds.

While these nutrients are essential for biological treatment processes (microorganisms need them to grow and break down organic matter), excessive discharge into water bodies causes eutrophication – a process where nutrient overload triggers rapid algal growth, which in turn depletes oxygen and kills aquatic life.

Biological characteristics of dairy effluent 🔗

The biological properties of dairy effluent are primarily defined by its microbial content and biodegradability.

Microbial load 🔗

Dairy wastewater may contain pathogenic microorganisms originating from contaminated raw milk or production processes. Common microbial contaminants include E. coli, Salmonella, and Listeria. The high organic content of the effluent provides an ideal growth medium for bacteria, so microbial populations can multiply quickly if the wastewater is not treated promptly. The Most Probable Number (MPN) test for coliform bacteria is commonly used to assess the microbial contamination level.

Biodegradability 🔗

The good news is that dairy effluent is generally highly biodegradable. The BOD-to-COD ratio (typically 0.4-0.8) confirms that a large portion of the organic load can be broken down by microorganisms. About 90% of the BOD and COD in whey, for example, is attributable to lactose, which is readily fermentable. This makes biological treatment – both aerobic and anaerobic – particularly effective for dairy wastewater. However, the presence of cleaning chemicals (especially CIP solutions containing strong acids, alkalis, and sanitisers) can temporarily inhibit or even kill the microbial populations responsible for biological treatment, requiring careful equalisation and buffering.

Factors that influence dairy effluent characteristics 🔗

No two dairy plants produce identical wastewater. Several factors determine the composition and strength of dairy effluent:

Type of product: Cheese production generates the most polluted wastewater due to whey discharge, while liquid milk pasteurisation produces comparatively lighter effluent. Butter and cream processing result in high-fat wastewater.

Water usage efficiency: Plants that use 2-3 litres of water per litre of milk processed produce more dilute effluent. Efficient operations using less than 1.5 litres per litre create more concentrated wastewater, which affects treatment system sizing.

CIP practices: The type, frequency, and concentration of cleaning chemicals used in CIP cycles significantly influence pH, COD, and chemical residue levels in the effluent. CIP wastewater is typically discharged at 12- or 24-hour intervals, causing sharp fluctuations in effluent quality.

Seasonal variation: Milk production peaks during certain months (summer in many regions), leading to higher wastewater volumes and stronger pollutant loads during those periods.

Plant management: Good housekeeping practices – reducing spills, recovering product before wash-down, and segregating waste streams – can dramatically reduce effluent strength and volume.

Why understanding these characteristics matters 🔗

Characterising dairy effluent is not just an academic exercise. It is the essential first step in designing a treatment system that actually works. Each parameter guides a specific treatment decision:

High BOD and COD indicate the need for robust biological treatment. Anaerobic systems (like UASB reactors) are often preferred for high-strength dairy effluent because they can recover energy as biogas while reducing organic load.

Elevated FOG levels demand pre-treatment with dissolved air flotation (DAF) or grease traps before biological stages.

Wide pH swings require an equalisation tank with pH correction to protect downstream biological processes.

High nutrient concentrations may necessitate tertiary treatment steps – such as chemical precipitation for phosphorus removal – to meet discharge standards and prevent eutrophication.

TSS and TDS levels determine whether simple screening and sedimentation are sufficient or whether advanced filtration technologies (membrane bioreactors, reverse osmosis) are needed for reuse-quality water.

Continuous monitoring of these parameters allows operators to detect changes in effluent quality early and adjust treatment processes before problems – like system upsets, permit violations, or environmental damage – occur.

Typical parameter ranges at a glance 🔗

Here is a summary of the key dairy effluent parameters and their typical ranges based on data from multiple studies and industry references:

Total Solids (TS): 1,000-4,000 mg/L

Total Suspended Solids (TSS): 100-1,500 mg/L

Total Dissolved Solids (TDS): 700-2,500 mg/L

BOD₅: 1,000-4,000 mg/L

COD: 1,500-6,000 mg/L

pH: 2-12 (commonly 6-9 after equalisation)

FOG: 0.2-2.88 g/L (varies by product type)

Total Nitrogen: 14-830 mg/L

Total Phosphorus: 9-280 mg/L

Turbidity: 100-800 NTU

Temperature: 17-25 °C

These values can vary significantly depending on plant type, product mix, and operational practices, which is why site-specific effluent characterisation is always recommended before designing a treatment plant.

What do you think? If dairy effluent is highly biodegradable, could smaller dairy cooperatives in rural India benefit from low-cost biological treatment systems like waste stabilisation ponds or constructed wetlands? And how might better in-plant water management practices reduce the pollution load before the wastewater even reaches the treatment plant?

References

1. https://clearfox.com/dairy-industry-wastewater/
2. https://dairyprocessinghandbook.tetrapak.com/chapter/dairy-effluent
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC5434364/
4. https://pubs.sciepub.com/jaem/2/1/4/
5. https://www.dairyfoods.com/articles/96638-advancing-dairy-wastewater-treatment
6. https://www.sciencedirect.com/science/article/abs/pii/S0959652619315410
7. https://www.waterindustryjournal.co.uk/dairy-wastewater-treatment-and-design
8. https://ecologixsystems.com/industries/dairy