Dairy Science & Technology · Dairy Engineering & Plant Operations · also Dairy Waste Management, Environmental Engineering for Dairy Plants
Characterising and treating dairy wastewater, sludge and emissions, and measuring the environmental footprint of milk.
In the exams: ICAR AICE subject 34 Unit 4 (BOD and COD in dairy waste) and subject 35b Unit 1 (EMS); ICAR AIEEA-PG group 16 (disposal of dairy waste)
Dairy plants are among the food industry's largest sources of high-strength wastewater, and the dairy sector as a whole carries a substantial greenhouse gas and water footprint. Effluent rich in milk solids can deplete oxygen in rivers, cause odours, and overload municipal sewage works; fat clogs sewers; nutrients cause eutrophication; and salts from cleaning can damage soils where effluent is used for irrigation. At the same time, dairy effluent is highly biodegradable and can yield biogas, and the water in milk itself can be recovered. Managing effluent and environmental performance is therefore both a legal obligation and an opportunity.
This manual covers the sources and characteristics of dairy effluent; the measurement of BOD, COD, fats, oils and grease and nutrients; discharge standards and permits; preliminary and primary treatment including screening, equalisation and dissolved air flotation; aerobic treatment by activated sludge and its variants; anaerobic treatment and biogas; nutrient removal and tertiary treatment; sludge handling and disposal; water recovery from cow water and condensate; life-cycle assessment and the carbon footprint of dairy products; and environmental management systems under ISO 14001. Utilities and energy are covered in dairy-plant-utilities-energy, refrigeration and refrigerant rules in dairy-refrigeration-cold-chain, CIP chemicals and water in cip-sanitation-hygienic-design, whey use in whey-dairy-byproducts, and general water and wastewater engineering in the sustainability track's water-wastewater-treatment; this manual applies those principles to dairy.
Postgraduate entrance and research-fellowship examinations in dairy science set BOD and COD of dairy waste, effluent treatment and environmental management systems, and dairy officer and food safety examinations test discharge standards and permits. Values and limits given here are representative; current permits, national standards and laboratory methods govern practice.
Dairy effluent is the wastewater a dairy plant discharges. Most of its pollution load is product that has been lost: milk, cream, whey, buttermilk, ice cream mix and other products washed out of tanks, pipes and equipment at the start and end of runs, at product changes, from spills, leaks and overflows, from separator and clarifier desludging, and from rejected or returned products. Cleaning solutions add detergents, caustic soda, acids and sanitisers; boiler blowdown, cooling-tower bleed, water treatment regeneration brine and condensates add salts and heat; and domestic sewage from canteens and toilets is often combined with the process stream. Whey from cheese and casein making is a very concentrated stream and should be kept separate and used as a product (see whey-dairy-byproducts) rather than discharged, since even a small proportion of whey entering the effluent can overwhelm a treatment plant. Stormwater from roofs and yards should be kept separate from process effluent so that it does not dilute and overload the treatment system in wet weather.
Because it consists largely of diluted milk components, dairy effluent is rich in biodegradable organic matter: lactose, which degrades quickly and can make the effluent acidic as it ferments; proteins, which degrade more slowly and release ammonia; and milk fat, which floats, clogs pipes and is slow to degrade. Typical raw effluent may have a biochemical oxygen demand of a few hundred to several thousand milligrams per litre, a chemical oxygen demand of roughly 1.5 to 2 times the BOD, high fats, oils and grease, suspended solids, nitrogen (from proteins and some cleaning agents) and phosphorus (from milk and phosphate-containing detergents). pH swings widely, from strongly alkaline during caustic cleaning to acidic during acid cleaning or after lactose fermentation, and temperature can be high. Flow and load vary greatly by hour of day, by day of week and by season, peaking during cleaning and at the end of production. Effluent volume is commonly one to several litres per litre of milk processed. Characteristics differ by product: cheese and casein plants have high loads if whey enters the drain; butter and ghee plants have high fat; milk powder plants produce large volumes of condensate and cleaning effluent.
Milk itself has a BOD of roughly 100,000 mg/L and a COD of roughly 200,000 mg/L, so each litre of whole milk lost to drain carries about 0.1 kg of BOD; whey has a BOD of about 30,000–50,000 mg/L. A plant that loses 1 per cent of 500,000 litres a day therefore sends about 5,000 litres of milk, or around 500 kg of BOD, to drain, equivalent to the daily organic load of thousands of people. Product loss is both a financial loss and an environmental burden, so the cheapest effluent treatment is prevention: dry cleaning of spills, product recovery by pigging, rinse recovery, accurate level controls to prevent overflows, separate collection of first rinses for animal feed or biogas, and good housekeeping. Many plants measure their product loss by comparing the COD load in effluent with the milk processed.
A structured waste minimisation programme begins with an effluent audit: flow and COD are measured at each major drain point and over each shift, and the results are mapped against production events such as tank emptying, product changeovers, CIP cycles and separator desludging. This normally shows that a few events cause most of the load. Typical measures then follow: draining tanks and lines fully before rinsing; recovering product from pipes by pigging or by pushing it out with water and collecting the mixed phase; collecting first rinses separately for animal feed, biogas or rework where permitted; fitting high-level alarms and interlocks to prevent tank overflows; collecting separator desludge in a tank instead of sending it to drain; returning rejected product to a dedicated route rather than the floor drain; repairing leaking seals and valves; and using conductivity or turbidity sensors to divert product-rich streams. Training operators to treat every litre to drain as lost money, and reporting COD per tonne of milk processed alongside production figures, keeps the programme alive.
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen consumed by microorganisms as they oxidise the biodegradable organic matter in a sample, under standard conditions. The usual test is BOD5 at 20 °C over five days, although some warm-climate jurisdictions specify BOD at 27 °C over three days, which gives similar values; a permit always names the test it uses. The sample is diluted with aerated dilution water containing nutrients and, if needed, a seed of microorganisms; dissolved oxygen is measured at the start and end of incubation in the dark; and BOD = (initial DO − final DO) ÷ dilution fraction. For example, if a sample is diluted 1 in 50 (fraction 0.02) and dissolved oxygen falls from 8.5 to 4.1 mg/L, BOD = 4.4 ÷ 0.02 = 220 mg/L. The depletion should be at least about 2 mg/L and leave at least about 1 mg/L, so several dilutions are set up. Nitrification can add to oxygen demand and is suppressed with an inhibitor when carbonaceous BOD alone is wanted. The BOD exertion over time follows approximately first-order kinetics: BOD_t = L0 (1 − e^(−kt)), where L0 is the ultimate BOD and k the rate constant.
Chemical oxygen demand (COD) measures the oxygen equivalent of organic matter that can be oxidised by a strong chemical oxidant — usually potassium dichromate in hot sulphuric acid with a silver sulphate catalyst, with mercuric sulphate added to mask chloride interference. It takes two to three hours rather than days, so it is used for daily process control, and the BOD to COD ratio indicates biodegradability: dairy effluent, with a ratio around 0.5 to 0.7, is readily biodegradable. Total organic carbon (TOC), measured instrumentally by oxidation and carbon dioxide detection, is a faster alternative and is used for online monitoring. Suspended solids are measured by filtering, drying at 103–105 °C and weighing; volatile suspended solids by igniting the residue at 550 °C, giving an estimate of organic and biomass content.
Fats, oils and grease (FOG) are measured gravimetrically after solvent extraction from an acidified sample, the solvent being evaporated and the residue weighed; methods specify the solvent. FOG matters in dairies because fat blocks sewers, coats biomass and floats in tanks. Nitrogen is measured as total Kjeldahl nitrogen (organic plus ammonia nitrogen, by digestion and distillation, the same principle as protein determination in milk), ammonia nitrogen, nitrate and nitrite. Phosphorus is measured as total phosphorus after digestion and as orthophosphate, commonly by colorimetry using the molybdenum blue method. Other routine parameters include pH, temperature, conductivity, total dissolved solids (raised by cleaning chemicals and regeneration brine), chlorides, and for discharge to land, sodium adsorption ratio. Sampling must be representative: because flow and load vary so much, flow-proportional composite samples over 24 hours are preferred over grab samples, and loads (kg/day) are calculated as concentration × flow — for example, 500 m³/day at a COD of 3,000 mg/L carries 1,500 kg of COD a day.
Dairy plants need permission to discharge effluent, and the permit sets limits that depend on where the effluent goes: to inland surface water, to a public sewer leading to a municipal treatment plant, to land for irrigation, or to the sea. Almost everywhere the legal basis is a water-pollution control or general environmental protection law, administered by a national or regional environmental regulator. The common structure has four parts. First, approval before building and permission before operating: many systems issue a single integrated permit, while others separate a consent to establish (granted on the plant's design, site and proposed treatment) from a consent to operate (granted once the plant is built and its treatment works), with periodic renewal. Second, standards: limits for pH, BOD or COD, suspended solids, fats, oils and grease, nitrogen, phosphorus and sometimes temperature, salinity and colour, set either as technology-based limits (what good treatment achieves), as water-quality-based limits (what the receiving river can absorb), or both, with the stricter applying. Some regulators publish dairy-specific standards; others write limits case by case into each permit. Third, monitoring: plants self-monitor at a stated frequency, keep records, report to the regulator, and in larger cases install continuous online monitoring linked to it. Fourth, enforcement: inspections, fines, closure orders and prosecution, and in water-scarce or sensitive catchments stricter conditions up to zero liquid discharge, where all water must be recovered and reused.
The same structure appears in different forms. In the European Union, the Industrial Emissions Directive requires permits based on best available techniques (BAT) for large food and milk installations, with BAT conclusions for the food, drink and milk industries setting associated emission levels, and the Urban Waste Water Treatment Directive governs discharges to sewers through municipal agreements. In the United States, the Clean Water Act regulates direct discharges through National Pollutant Discharge Elimination System (NPDES) permits, which include effluent limitation guidelines for dairy products processing, and indirect discharges through pretreatment standards and local limits set by the sewer authority. In India, the water and environment protection acts require a consent to establish and a consent to operate from a state board, and the dairy-specific standard has used pH 6.5–8.5, BOD (3 days at 27 °C) of 100 mg/L and oil and grease of 10 mg/L for discharge to surface water. In New Zealand, regional councils grant resource consents for discharges to water and to land, which matters because much dairy effluent there is irrigated onto pasture. Many countries also charge trade effluent fees based on volume and strength (often COD and suspended solids), which give plants a direct financial reason to reduce load. Permits also cover monitoring frequency, reporting, sludge disposal, odour, noise and spill response. Limits and test methods change, so the current permit and national standard always govern practice.
Discharge to land for irrigation is common in rural dairies and is regulated for hydraulic load, organic load, nutrients, salinity and sodium, since high sodium from caustic cleaning and brine can destroy soil structure. Discharge to a municipal sewer usually requires pretreatment to remove fat, balance pH and flow, and sometimes reduce organic load, because a large dairy can be the biggest single load on a town's treatment plant. Whichever route is used, the plant must know its flows and loads, keep monitoring records, and maintain contingency plans for spills and treatment failures.
Treatment begins by removing coarse material. Screens — bar screens, rotary drum screens and wedge-wire screens with openings of a millimetre or so — remove pieces of cheese curd, packaging, labels and debris that could block pumps and pipes. Fat traps or grease interceptors let fat float and be skimmed in small plants, although in larger plants they are replaced by flotation units. Grit chambers are seldom needed in dairies unless yard runoff enters the system.
Because flow, load, pH and temperature fluctuate widely, an equalisation (balancing) tank is the heart of a dairy effluent plant. It stores effluent for several hours to about a day, mixing it so that the downstream processes receive a steady flow and composition. Alkaline and acidic cleaning effluents partly neutralise each other in the tank, and further pH correction is applied with acid, caustic, lime or carbon dioxide before biological treatment, which needs a pH roughly between 6.5 and 8.5. The tank is mixed or aerated to prevent settling and septic conditions, but lactose can begin to ferment, lowering pH; some plants exploit this as pre-acidification for anaerobic treatment. Equalisation also allows a spill of concentrated product to be diluted or diverted to a separate tank, protecting the biological stage. The volume needed is found from a cumulative flow curve: the maximum difference between the cumulative inflow and a steady outflow line.
Dissolved air flotation (DAF) is the main primary treatment for dairy effluent because it removes fat and suspended solids that would otherwise overload biological treatment. Part of the clarified effluent is recycled, saturated with air under pressure (typically 4–6 bar), and released into the flotation tank, where the pressure drop forms fine bubbles that attach to fat globules and particles and float them to the surface as a sludge blanket, which is skimmed off. Coagulants (such as ferric chloride, aluminium salts or polyaluminium chloride) and polymer flocculants are added beforehand, usually at a controlled pH, to break emulsions and flocculate proteins and fat, greatly improving removal. A well-operated chemical DAF can remove most of the FOG and suspended solids and a substantial part of the COD, since much of the COD in dairy effluent is in colloidal fat and protein. The DAF float is a concentrated, fatty sludge that is often sent to anaerobic digestion or rendering. The air-to-solids ratio is a key design parameter. Primary settling tanks are less effective for dairy effluent because fat floats and fine protein flocs settle poorly.
In the activated sludge process, effluent is mixed in an aeration tank with a suspension of microorganisms (the mixed liquor) and supplied with oxygen by diffused air or mechanical aerators. Bacteria use the organic matter for energy and growth, forming flocs that are then separated in a secondary clarifier; most of the settled sludge is returned to the aeration tank to keep a high biomass concentration (mixed liquor suspended solids, typically 3,000–5,000 mg/L in conventional plants), and the surplus is wasted. Key design and operating parameters are hydraulic retention time; sludge age or solids retention time (the average time biomass stays in the system, commonly 10–30 days for dairy plants that need stable operation and nitrification); the food-to-microorganism (F/M) ratio, kilograms of BOD applied per day per kilogram of MLSS; dissolved oxygen (around 2 mg/L); and the sludge volume index (SVI), the volume in millilitres occupied by one gram of sludge after 30 minutes' settling, which indicates settleability. For example, an aeration tank of 1,000 m³ at an MLSS of 4,000 mg/L holds 4,000 kg of biomass; if it receives 600 kg of BOD a day, F/M = 0.15 per day, a typical extended-aeration value.
Dairy effluent is easily degraded but prone to problems. Its high, readily available carbohydrate and low nitrogen and phosphorus relative to organic load in some streams, combined with low dissolved oxygen, low F/M or fat, favour filamentous bacteria that cause bulking — sludge that settles poorly and is lost from the clarifier — and foaming. Fat coats flocs and reduces oxygen transfer. Shock loads from spills cause oxygen depletion and failure. Nutrients may need to be added if the BOD:N:P ratio falls short of roughly 100:5:1. Remedies include good equalisation, efficient fat removal, adequate aeration, selectors (small, high-load contact zones that favour floc-formers over filaments), chlorination of return sludge in emergencies, and control of F/M and sludge age. Oxygen demand is high, so aeration is the main energy cost of aerobic treatment, often more than half of the plant's power use.
Extended aeration uses a long retention time and low F/M, producing less, more stable sludge, and suits small and medium dairies. Sequencing batch reactors (SBRs) carry out filling, reaction, settling and decanting in one tank in timed cycles, giving flexibility for variable dairy flows and enabling nitrogen and phosphorus removal through anoxic and anaerobic phases. Membrane bioreactors (MBRs) replace the clarifier with submerged microfiltration or ultrafiltration membranes, allowing high MLSS, a small footprint and a high-quality, solids-free effluent suited to reuse, at the cost of energy and membrane maintenance. Moving bed biofilm reactors (MBBRs) carry biomass on suspended plastic carriers, and trickling filters and rotating biological contactors are older attached-growth processes. Aerated lagoons and oxidation ponds are cheap where land is available, as in many rural dairies, but need large areas and can smell if overloaded. General treatment principles are set out in the sustainability track's water-wastewater-treatment.
The oxygen an activated sludge plant must supply covers the carbonaceous BOD removed, endogenous respiration of the biomass and, where nitrification occurs, about 4.6 kg of oxygen per kilogram of ammonia nitrogen oxidised. A simple estimate for an extended-aeration dairy plant is roughly 1.0–1.5 kg of oxygen per kilogram of BOD removed, plus the nitrification demand. Aerator capacity is then sized from the oxygen transfer efficiency of the chosen diffusers or surface aerators under field conditions, which is lower than the clean-water rating, especially with fat present. Operators control the plant daily through dissolved oxygen (often with variable-speed blowers to save energy), sludge wasting to hold the target sludge age, the 30-minute settling test and SVI, microscopic examination of flocs and filaments, and effluent quality. Records of these parameters, read together with production and spill events, allow problems to be traced and corrected before consent limits are breached.
In anaerobic treatment, microorganisms break down organic matter without oxygen in four stages: hydrolysis of fats, proteins and carbohydrates; acidogenesis to volatile fatty acids; acetogenesis to acetate, hydrogen and carbon dioxide; and methanogenesis, in which methanogenic archaea produce methane. The products are biogas — typically 60–70 per cent methane and 30–40 per cent carbon dioxide with traces of hydrogen sulphide — and a small amount of sludge, since anaerobic bacteria convert most of the organic carbon to gas rather than biomass. Theoretically 0.35 m³ of methane at standard conditions is produced per kilogram of COD removed. Methanogens are slow-growing and sensitive to pH (best near 6.8–7.4), temperature changes, volatile fatty acid build-up, ammonia, sulphide and oxygen, so the process needs good control. Most dairy reactors operate in the mesophilic range (about 30–40 °C).
High-rate anaerobic reactors retain biomass so that they can treat high loads in small volumes. Upflow anaerobic sludge blanket (UASB) reactors pass effluent upward through a dense blanket of granular sludge, with a three-phase separator at the top to separate gas, liquid and solids; expanded granular sludge bed (EGSB) and internal circulation reactors are taller, higher-rate versions. Anaerobic filters retain biomass on media, and anaerobic contact processes use a digester followed by a settler with sludge return. Covered anaerobic lagoons are low-cost options for rural plants. Dairy effluent is well suited to anaerobic treatment because of its high, readily degradable COD, but its fat can inhibit granulation and float sludge, so fat is removed first (by DAF) and the float digested separately, or reactors designed for fat are used. Whey and high-strength streams are particularly attractive feeds for biogas. Anaerobic treatment alone rarely meets discharge limits, so it is followed by aerobic polishing. Its advantages are low energy use (no aeration), energy recovery as biogas, low sludge production and small footprint; disadvantages are sensitivity, slow start-up and incomplete nutrient removal.
Biogas is used in boilers, in combined heat and power engines, or upgraded to biomethane (by removing carbon dioxide and hydrogen sulphide) for injection into gas grids or as vehicle fuel. Hydrogen sulphide must be removed — by biological scrubbing, iron-based scrubbers or air dosing — to protect engines and boilers from corrosion. A kilogram of COD removed anaerobically gives about 0.35 m³ of methane, whose energy content is about 35.8 MJ per cubic metre, so 1,000 kg of COD removed a day yields about 350 m³ of methane, roughly 12,500 MJ, or around 3,500 kWh of primary energy, of which a CHP engine might convert about 35–40 per cent to electricity. Biogas and its use are also discussed in dairy-plant-utilities-energy.
Anaerobic reactors are monitored more closely than aerobic plants because failure can build up unnoticed. The key indicators are pH, volatile fatty acids (VFA), alkalinity, the ratio of VFA to alkalinity, biogas flow and methane content, and COD removal. When the acid-forming stages outpace methanogenesis — after an organic overload, a temperature drop or a toxic shock — VFAs accumulate, consume alkalinity and finally lower pH, which further inhibits the methanogens in a vicious circle. A rising VFA-to-alkalinity ratio, falling gas production or falling methane content give early warning, and the response is to reduce the load, add alkalinity (sodium bicarbonate or lime) and restore temperature. Pre-acidification tanks upstream of UASB reactors partly convert lactose to acids under controlled conditions, protecting granular sludge. Dairy effluents can be short of trace elements such as nickel, cobalt and iron, which methanogens need, and some plants dose them. Start-up of a new reactor is slow, often taking weeks to months, and is faster with seed granules from an existing reactor.
Dairy effluent contains nitrogen from proteins and some cleaning agents (such as nitric acid). In biological treatment, organic nitrogen is converted to ammonia, and ammonia is then oxidised to nitrite and nitrate by autotrophic nitrifying bacteria (nitrification), which grow slowly, need a long sludge age, enough oxygen and alkalinity, and are sensitive to low temperature and toxic shocks. Nitrification consumes about 4.6 g of oxygen and 7.1 g of alkalinity (as calcium carbonate) per gram of ammonia nitrogen oxidised. Nitrate is then removed by denitrification, in which heterotrophic bacteria under anoxic conditions (no free oxygen) use nitrate instead of oxygen to oxidise organic carbon, releasing nitrogen gas. In the common Modified Ludzack–Ettinger arrangement, an anoxic tank precedes the aerobic tank and nitrate-rich mixed liquor is recycled to it, so that raw effluent provides the carbon source; dairy effluent's high carbon content makes denitrification easy.
Phosphorus causes eutrophication of lakes and rivers and is often tightly limited. Dairy effluent phosphorus comes from milk and from phosphate-based detergents, so substituting phosphate-free cleaning agents reduces it at source. It can be removed chemically by precipitation with iron or aluminium salts (in the DAF, the aeration tank or a tertiary stage) or biologically by enhanced biological phosphorus removal (EBPR), in which polyphosphate-accumulating organisms, cycled between anaerobic and aerobic zones, take up more phosphorus than normal growth requires and are removed in waste sludge.
Tertiary treatment polishes biologically treated effluent to meet strict discharge limits or for reuse. Options include sand or multimedia filtration and disc filters to remove residual suspended solids; activated carbon for residual organics and colour; disinfection by chlorination, ultraviolet light or ozone; membrane filtration (ultrafiltration, then reverse osmosis) to produce water suitable for reuse; and constructed wetlands, which are used in some rural plants for final polishing. Reverse osmosis produces a concentrated reject stream containing salts, which must be managed — in zero liquid discharge systems it is evaporated and crystallised, leaving solid salts. The choice depends on the receiving water, regulations and reuse plans.
Effluent treatment produces several sludges: DAF float (fatty, high in organic matter, often 3–10 per cent solids, and putrescible), waste activated sludge (biomass, typically 0.5–1.5 per cent solids as wasted, produced at roughly 0.3–0.5 kg of dry solids per kilogram of BOD removed in aerobic plants), anaerobic sludge (smaller amounts, more stable), and chemical sludge from coagulation and phosphorus precipitation. Sludge volume is dominated by water, so thickening and dewatering are essential to reduce handling and disposal costs. Since dry solids mass is constant, the volume falls sharply as solids content rises: 100 m³ of sludge at 1 per cent solids contains 1 tonne of solids, and dewatering it to 20 per cent solids leaves about 5 m³.
Thickening by gravity, flotation or rotary drum thickeners raises solids content to a few per cent; dewatering by belt presses, screw presses, decanter centrifuges or filter presses, with polymer conditioning, raises it to roughly 15–25 per cent or more; drying beds are used in hot, dry climates and solar dryers in some plants. Stabilisation reduces odour and pathogens — by anaerobic digestion (which also yields biogas), aerobic digestion, liming or composting. Disposal routes include land application as a soil conditioner and fertiliser (valuable for its nitrogen and phosphorus but regulated for pathogens, heavy metals, odour and nutrient loading), composting with agricultural residues, co-digestion in biogas plants, and, where nothing else is possible, landfill or incineration. DAF float with high fat content may go to rendering or biogas plants. Environmental regulators control sludge from effluent treatment: sewage and industrial sludge rules set limits for land application, and some sludges must go to authorised disposal facilities.
Sludge management is often the second-largest operating cost of a dairy effluent plant after aeration energy, so the mass balance matters. The dry solids produced each day equal the sludge yield multiplied by the BOD removed, plus chemical solids from coagulants and DAF float solids. For example, an aerobic plant removing 1,000 kg of BOD a day at a net yield of 0.4 kg dry solids per kilogram produces about 400 kg of dry solids a day; wasted at 1 per cent solids that is 40 m³ of liquid sludge, and dewatered to 20 per cent it becomes about 2 m³ of cake. Polymer dose, power for dewatering, haulage and gate fees are all charged per tonne, so improving cake dryness by a few percentage points brings large savings. Anaerobic pretreatment cuts sludge production sharply because far less of the organic load becomes biomass.
Milk is about 87 per cent water, and whey about 93 per cent. When a dairy concentrates milk or whey in evaporators, or concentrates by reverse osmosis, it removes large volumes of this water. In evaporators the vapour is condensed — after giving up its heat in the next effect or in vapour recompression (see evaporation-drying-dairy) — to produce condensate, called cow water or milk condensate. A powder plant can produce more water from milk than it consumes from wells or mains. Cow water is not pure: it carries volatile organic compounds, traces of entrained product and ammonia, giving it a COD of perhaps tens to a few hundred mg/L, and it is warm, which favours microbial growth if stored. It is therefore polished before reuse.
Typical polishing trains include reverse osmosis (often a 'polisher' RO with high rejection), sometimes with activated carbon or biological treatment for volatile organics, followed by ultraviolet disinfection or chlorination, with continuous monitoring of conductivity, TOC or turbidity to divert off-specification water. Polished cow water is used for boiler feed (where its low hardness is an advantage, although organics must be low to prevent foaming), cooling-tower make-up, CIP pre-rinses and caustic make-up, and in some plants, after validation and approval, for final rinses and other uses requiring potable quality. Food safety rules govern such reuse: the Codex Alimentarius has guidance on water reuse in food production, and national regulators set conditions. Reverse osmosis permeate from milk or whey concentration (see membrane-processing-dairy-ingredients) is a similar resource. Reuse of treated effluent, generally for non-product uses only, completes the picture, and some plants recover enough water to become net exporters for local irrigation.
Life-cycle assessment (LCA) quantifies the environmental impacts of a product over its life cycle, from raw materials to disposal ('cradle to grave') or to the factory gate ('cradle to gate'). It is standardised in ISO 14040 and ISO 14044 in four phases: goal and scope definition (including the functional unit and system boundary), life-cycle inventory (collecting inputs and outputs), life-cycle impact assessment (converting inventory into impact categories such as climate change, eutrophication, acidification, water scarcity and land use), and interpretation. For dairy, the functional unit is commonly a kilogram of fat-and-protein-corrected milk (FPCM) at the farm gate, standardised to 4.0 per cent fat and 3.3 per cent protein so that milks of different composition can be compared, or a kilogram of product. Allocation — dividing the impacts of a multi-output system between products — is a key methodological choice: on the farm, between milk and meat from culled cows and calves; in the plant, between cheese and whey, or skim and cream. The International Dairy Federation's guide to the carbon footprint of dairy products recommends physical (biological) allocation on the farm and allocation by milk solids in processing, though other methods are used.
For most dairy products, the farm dominates the carbon footprint — commonly around 80–90 per cent of cradle-to-gate greenhouse gas emissions — through enteric methane from cattle rumen fermentation, nitrous oxide from manure and fertilised soils, methane from manure storage, feed production, and land-use change for feed. Processing, packaging, transport, retail and consumer use make up the rest; within processing, energy for heating, refrigeration and drying, and refrigerant leakage, are the main contributors. Products that concentrate milk solids have higher footprints per kilogram: about 10 litres of milk go into a kilogram of hard cheese, so cheese has a footprint many times that of milk per kilogram, though not per unit of protein. Greenhouse gases are expressed as carbon dioxide equivalents using global warming potentials over 100 years (methane roughly 27–30 and nitrous oxide roughly 273 in recent IPCC assessments). Farm footprints vary widely between systems and regions with yield per cow, feed efficiency and manure management.
Processors influence emissions directly through energy efficiency, renewable energy, heat pumps, refrigerant choice, product loss reduction and packaging, and indirectly by working with farmers on feed efficiency, herd health, manure management, biogas and additives that reduce enteric methane. Greenhouse gas accounting follows the GHG Protocol's scopes: Scope 1 (direct emissions, such as boilers and refrigerant leaks), Scope 2 (purchased electricity and heat) and Scope 3 (value chain, dominated for dairy by purchased milk). Science-based targets, product carbon labels, and industry frameworks such as the Dairy Sustainability Framework guide action. Water footprint and biodiversity impacts are increasingly assessed alongside carbon. LCA results depend strongly on method choices, so comparisons need consistent methods and transparent assumptions.
An environmental management system (EMS) is a structured way for an organisation to manage its environmental responsibilities. ISO 14001 (current edition 2015) specifies requirements for an EMS using the Plan–Do–Check–Act cycle and the high-level structure common to ISO management standards, which makes it easy to integrate with ISO 9001 (quality), ISO 22000 or FSSC 22000 (food safety), ISO 45001 (occupational health and safety) and ISO 50001 (energy). Its main elements are: understanding the organisation's context and the needs of interested parties; leadership commitment and an environmental policy; identifying environmental aspects (activities that interact with the environment, such as effluent discharge, energy use, refrigerant use, packaging waste and odour) and their impacts, and determining which are significant; identifying compliance obligations (legal and other requirements); setting objectives and planning actions; resources, competence, awareness and communication; operational control, including life-cycle perspective; emergency preparedness and response (for example ammonia releases or effluent spills); monitoring, measurement and evaluation of compliance; internal audit; management review; and continual improvement, including handling nonconformities and corrective action.
In a dairy, significant aspects typically include effluent load and volume, water consumption, energy use and greenhouse gas emissions, refrigerant leaks, packaging and solid waste, odour from effluent plants, noise, and the risk of milk or chemical spills reaching drains or watercourses. Objectives might be to reduce COD sent to the effluent plant per tonne of milk, water use per litre of milk, energy per tonne of product, or landfill waste to zero. Operational controls include spill procedures, chemical storage with bunding, effluent plant operating procedures, waste segregation and contractor control. Certification by an accredited third-party body is voluntary but often required by customers, and audits examine both the system and evidence of performance and legal compliance. The European Eco-Management and Audit Scheme (EMAS) builds on ISO 14001 with additional public reporting. An EMS does not set performance levels itself; it provides the framework through which the organisation meets its legal obligations and its own targets, so performance depends on the ambition and follow-through of management.
A student uses this subject to calculate BOD, loads, F/M, sludge volumes and biogas yields, explain DAF, activated sludge and anaerobic treatment, and describe discharge standards, LCA and ISO 14001. It supports AICE Dairy Chemistry and Microbiology questions and AIEEA-PG dairy waste questions.
An environment or effluent plant manager five years in runs the ETP, controls DAF chemicals and aeration, troubleshoots bulking and reactor upsets, keeps consent records and reports to the Pollution Control Board, tracks product loss and water use, and runs the ISO 14001 system.
A plant head or sustainability director sets water, effluent and carbon targets, chooses between anaerobic, aerobic, MBR and ZLD investments, negotiates permits, and leads farm-level emission programmes with milk suppliers.
A professor or research director works on high-rate anaerobic treatment of dairy wastes, nutrient recovery, water reuse and LCA methods for dairy products, and supervises research and examines dairy chemistry and microbiology.
This manual does not yet give: the current text of each country's dairy effluent standards, permit conditions and recent amendments; full laboratory procedures for BOD, COD and FOG; detailed design of activated sludge, UASB and DAF units; regulated limits for land application of sludge in each country; or quantitative carbon footprint values for specific dairy products and regions. Concentrations, yields, percentages and global warming potentials are representative and should be checked against current sources.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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