From Cow To Cheese – Wastewater Treatment In The Milk Processing Industry

Wastewater is a permanent by-product of the milk processing industry. It is produced in almost every process step: slurry is produced during milk production. In combination with wash water, it is suitable for spreading on fields. However, since milk is a foodstuff, hygiene is a crucial aspect of processing. Therefore, regular cleaning of milking machines and other processing technologies is mandatory.

Wastewater is also produced during processing into dairy products such as cheese, yoghurt, or butter. Without treatment, this must not be discharged into the environment, as it can otherwise cause irreparable damage to nature.

The milk-processing industry distinguishes between two main sectors: Firstly, there are the milk producers who provide the raw product. In addition to liquid manure, this mainly results in cleaning water. On the other hand, there are milk-processing companies, which produce very different types of wastewater's due to the different products. Hybrid companies, which process the milk at the point of origin, play a subordinate role because they are rare and usually process milk in smaller quantities and thus produce less wastewater.

Slurry treatment as an indicator of sustainable environmental protection

Slurry is a natural fertiliser made up of the faeces and urine of farm animals. Cows and cattle cover a considerable part of this, as they produce around 23 m³ of manure annually. This mainly contains bound nitrogen, potassium nitrate, sodium nitrite, ammonium, magnesium sulphate and phosphorus. This makes it very suitable as a fertiliser. In most European countries, farmers are required to spread the manure produced on fields. In this way, they keep the nutrients in circulation and ensure better plant growth in a natural way.

In recent years, however, a trend has developed that stands in the way of this practice. More and more large-scale livestock farms are producing more manure than they are allowed to spread on their fields. This results in an overload of nitrogen, which causes lasting damage to the soil. In addition, farmers risk contaminating groundwater.

Farmers spread between 20 and 30 m³ of manure per hectare. If the animals kept exceed this limit, the farmer is only allowed to spread part of it or must arrange a removal, which is usually very expensive. To prevent this challenge, pre-treatment of the manure is an alternative. The processes lower the nitrogen content so that the same amount of slurry can be spread on the fields with the permitted nutrient content.

The wastewater characteristics

During the processing of milk, different types of wastewaters are produced, depending on the processing procedure and the end product. Even during the transport of milk, regular cleaning of the tank trucks is essential. Condensates are produced during further processing into homogenised milk. The further processing of milk into butter, cheese or yoghurt removes water from the original product, which still has many dissolved substances.

After processing into dairy products, the wastewater has the following average values:

• BOD₅: 1,800 mg/l
• COD: 3,500 mg/l
• Total dissolved solids: 400 mg/l
• pH value: 5.5
• Ammonium: 25 mg/l
• Nitrate: 6 mg/l
• Phosphorus: 36 mg/l

The four options for wastewater treatment

Essentially, there are four options to deal with the wastewater produced. The first option is collection in a storage tank and regular removal. The second option is pre-treatment followed by discharge into a municipal wastewater treatment plant. The third option is to treat the wastewater more thoroughly and then discharge it into a receiving water body such as a stream, lake or septic tank. The fourth option is to treat the wastewater so thoroughly that it is suitable for reuse in the company's own process.

Removal. The wastewater is usually stored in tanks. At regular intervals, disposal companies collect the wastewater by tanker truck and transport it to the nearest municipal wastewater treatment plant. This method is very cost-intensive and is rarely used.

Indirect discharge. Indirect discharge into a municipal wastewater treatment plant usually requires pre-treatment of the wastewater. Screening of coarse solids that are too heavy to enter the municipal treatment plant via the sewer system is obligatory.

Depending on the degree of pollution of the wastewater, the municipal wastewater treatment plant sets a fee for the treatment. If the company can reduce certain pollution parameters in the wastewater in advance, the treatment fees incurred also decrease. Pre-treatment in the company is usually cost-efficient.

Direct discharge. Direct discharge is the discharge of wastewater into a receiving water body. In this case, governments or regional authorities set limits for the individual parameters. To comply with these, more stringent wastewater treatment is mandatory. Not only is it necessary to screen coarse contaminants, but also to reliably remove oils and greases as well as other suspended and dissolved substances.

Reuse. The aim of this type of wastewater treatment is to produce wash water that enables the CIP process to be carried out in-house. To achieve this, several process steps are usually necessary, which, connected one after the other, enable increasingly finer wastewater treatment. Biological treatment processes and ultrafiltration are particularly suitable for this in the final steps. This process conserves the company's water consumption during operation.

Which processes are suitable for wastewater treatment in milk-processing companies?

To ensure adequate wastewater treatment for milk-processing companies, two treatment processes are essentially suitable. One is dissolved air flotation, the other is biological wastewater treatment using a fixed-bed reactor.

Dissolved air flotation. Dissolved air flotation is a chemical-physical process. The system doses flocculants into the wastewater. These have two main functions: on the one hand, they agglomerate the suspended substances into larger flocs, and on the other hand, they precipitate the dissolved substances in the wastewater to bind them as well. In the inlet to the flotation unit, the wastewater is supersaturated with oxygen by means of pressure, which is taken back in the reactor.

During this expansion, the oxygen dissolves out of the wastewater, creating extremely fine bubbles. These are so small that they rise at around 4 metres per hour. Thus, they can carry the bound solids upwards. At the surface, they collect as sludge and can thus be separated from the clear water.

Fixed bed biological reactor. The fixed bed biological reactor is a biological process in which microorganisms present in the wastewater treat it. The fixed bed material usually consists of net tubes, which have a high specific surface area. This is between 100 and 300 square metres per cubic metre.

The net tubes consist of a special growth material on which the microorganisms can settle particularly easily. An integrated aeration system supplies them with oxygen. This allows them to grow and respire the biodegradable substances in the wastewater. In this way, activated sludge is produced, which sinks to the bottom of the plant and supports further treatment.

Sludge treatment as an additional component of industrial wastewater treatment

Both processes, dissolved air flotation and fixed bed reactor, produce sludge that concentrates the separated contaminants. There are three different types of sludge: Primary sludge, secondary sludge, and DAF sludge. Primary sludge is coarse solid matter that sinks to the bottom in equalisation basins.

Secondary sludge (or activated sludge) is produced during biological wastewater treatment and contains concentrated microorganisms and contaminants from the wastewater. DAF sludge is distended suspended matter, which is composed of non-dissolved substances as well as polymers.

Sludge is a waste product of wastewater treatment that must be removed at regular intervals. Depending on the application and collection capacity, the removal intervals are shorter or longer, but on average industrial sludge only needs to be removed about once a year. This removal usually causes considerable costs – especially if there is no possibility of storage.

One way to reduce these costs is sludge treatment. This involves dewatering the sludge to reduce its volume. The dry matter content of industrial sludge is between 5 and 20%, so dewatering brings a significant saving in volume. Dewatering of slurry is also possible in this case. A frequently used solution is the so-called sludge screw press.

The core of this consists of rotating plates that transport the sludge forward. The sludge runs over a grid with a very small gap size. While the water can flow off through the grid, the screw transports the solids further. The dried solids are collected in a container at the end of the screw, while the separated water can be returned to the treatment plant.

The sludge produced in the milk-processing industry hardly differs from other sludge from industrial wastewater treatment. The wastewater contains floating fats and oils, which are bound and discharged via a DAF. For all other organic contaminants, biological wastewater treatment is suitable, which produces secondary sludge. Thus, the sludge hardly differs from that from the entire food and beverage industry as well as other branches in which organic contaminants are present in the wastewater.

From the field – wastewater treatment at one of the largest organic milk producers in Germany

PPU Umwelttechnik GmbH received an enquiry from one of the largest producers of organic milk – Hofgut Eichigt in central Germany. As the company expanded its capacities, the former treatment plant reached its load limits. The current wastewater volume is 160 m³/day. This includes 90 m³ of cleaning wastewater, 20 m³ of washing water from the milking carousel and 50 m³ of washing water from milk filling.

The wastewater is heavily polluted slurry and results from the daily cleaning work in the stalls. Milk is also a significant component of the wastewater, as milking machines are cleaned, but leakages also always occur during decanting into tanks or during further processing.

The inflow values from these three wastewater sources are on average:

• COD: 2,330 mg/l
• BOD_5max: 1,750 mg/l
• NH₄N: 25 mg/l
• NO₃N: 10 mg/l
• P: 25 mg/l

The effluent values to be achieved were defined as follows:

• COD: 75 mg/l
• BOD₅: 25 mg/l
• NH₄N: 10 mg/l
• N_total: 18 mg/l
• P: 2 mg/l

The customer's objective was to discharge the treated wastewater into a catch basin located on the company's premises.

To bridge the construction of the wastewater treatment plant, PPU Umwelttechnik GmbH initially offered a container-based treatment plant for lease. This plant contained all the components that would also be used in the final plant.

Therefore, the customer had the opportunity to test the treatment performance in advance. The delivery and commissioning of the leasing plant was limited to two days. After that, the construction of the actual treatment plant began.

The solution for this project was an underground sewage treatment plant in ten concrete tanks. The wastewater passes through a series of treatment steps. The first three tanks are respectively a mixing tank, a balancing tank, and a buffer tank. Since the wastewater is produced very irregularly, it is collected there to be passed on to the biological treatment stage at regular intervals.

Since these tanks are aerated – primarily to prevent the wastewater from thickening – initial biological treatment takes place there. Screening takes place between the mixing tank and the equalisation tank. A screening screw first filters all coarse components from the wastewater that are larger than three millimetres. These must not be allowed to enter the system, as they could otherwise lead to blockages.

The next five tanks contain the fixed-bed reactors for biological wastewater treatment. The growth area of the fixed-bed material increases from tank to tank from 100 to 150 to 200 m²/m³. This allows for increasingly thorough treatment.

The phosphorus is eliminated by downstream precipitation. The resulting sludge is separated by a lamella separator in the last tank. Afterwards, the wastewater is suitable for direct discharge into a receiving water body.