Slaughterhouse wastewater treatment plant


Slaughterhouse wastewater cannot be often discharged in the receiving body or in public sewage without a treatment.
Waste waters have not always the same characteristics due to different types of treated meat as beef, horse meat, sheep meat, poultry, etc.
There is an important difference between slaughtering house and a processing meat factory, especially for wastewater amount and pollutant concentrations.


Main pollutant source for slaughtering process are blood, faeces and urine, wash water (carcasses, floors, utensils, etc.), food residues mixed in stomachs and offal waste. The main pollutants are therefore organic and biodegradable (BOD, COD, N, P) and wastewater contains a high level of suspended solids. Generally there are no toxic substances.
The standard wastewater treatment sizing is based on the amount of treated meat.
Slaughter requires a large amount of water which ranges from 6 up to 36 cubic meters per tonne (live cattle), and pollutant can range from 6.5 up to 23.5 kg per tonne (U.S. Department of Health).
For bovine slaughtering, the typical pollutant concentrations can range as follow:

  • flow: 6 – 36 m3/ton
  • BOLD: 900 – 2200 mg/l
  • suspended solids (SS): 650-930 mg/l
  • organic nitrogen: 120 and 330 mg/l

The pollution contribution coming from a cow is approximately equal to: two and a half times a pig, a calf or a sheep.
For poultry available data in literature are less accurate, as for below, (defined for each animal):

  • flow: 12 – 90 l
  • BOLD: 13 – 19 g
  • suspended solids (SS): 7 – 13 g
  • grease: 0.6 g

Slaughterhouse wastewater is composed on several organic compounds, both in solution and in suspension, regardless of type of meat processed.
Pollutants have an organic matter similar to domestic wastewater. The important differences are pollutant concentrations and nutrients ratio.
Other kind of pollutants, not coming from cattle slaughtering, can be added: cleaning product (detergents and disinfectants) or other additives for processing (for example, table salt used in cooking tripe).

System layout

Slaughterhouses and household wastewater can be treated together when the industrial component is relatively low.
A good mixing and an equalization volume are required.
In most cases, a specific wastewater treatment plant is required upstream of the public sewer.
Pre-treatment, secondary treatment and tertiary treatment are required according to effluent quality.
The first step is always a mechanical treatment, followed by a biological treatment (aerobic or anaerobic), chemical coagulation and other tertiary treatments.
The meat processing waste water has a much higher content of fat than slaughter: it is recommended two different pre-treatment line.

Mechanical treatment

Mechanical treatments are screening, fat flotation and sedimentation.
Rotating and vibrating screen is made with 3 – 6 mm mesh.
Fat removal is obtained by flotation in a basin as grease trap or by Dissolved Air Flotation (DAF) with retention times from 20 to 30 minutes.
Sedimentation requires retention time of approximately 4 hours.
The spillway should be designed with a lower hydraulic load (1-2 order of magnitude) than conventional systems. For example: in domestic wastewater it ranges from 8 up to 15 m3/m per hour wile in the case of slaughtering wastewater it shall not exceed 0.55 m3/m per hour.
Primary sedimentation is usually installed as an optional upstream of an aerobic compartment instead is not required upstream of an anaerobic compartment.

Biological treatment

Activated sludge plant is the most used solution, even if, it requires high aeration times, plenty of air blown and a high recirculation rate. It can reach a BOD removal efficiency even above 95%.
There are no particular restrictions on air insufflation methods but the minimum concentration of dissolved oxygen (DO) must be at least 0.5 mg/l.
A denitrification zone is added for nitrogen removal, near to the oxidation tank.

Slaughterhouse wastewater aerobic treatment plant layout

Slaughterhouse wastewater treatment layout aerobic digestion

Anaerobic systems provide good results when combined with aerobic steps. They reach results with yields of 95%. Trickling filters are rarely used (biologic filter). They are often preceded by an anaerobic treatment if wastewater is not diluted with a large amount of domestic waste water.
The filter media must have a large free passages. A high recirculation ratio and a low organic load are required for an efficiency effluent (for example 0.32 to 0.45 kg BOD per m3 of filter media per day).

Slaughterhouse wastewater trickling filters layout

Slaughterhouse wastewater treatment layout trickling filters

Chemical coagulation processes and sedimentation are both a tertiary phase or, in some cases, a single treatment.
Aluminium sulfate, ferrous sulfate and ferric chloride are used for chemical precipitation.
Ferric chloride and lime obtain a pH correction (range 10-11) and a good COD reduction.
For an high quality effluent (I.e. 20 mg/l BOD) a further tertiary treatment can be added. For example rapid or slow sand filters, micro screen or phytoremediation plant.


Blood is a significant contribution, it has a BOD of about 100 mg / l and it contains 20% of solid substances.
Blood is usually recovered in a separate line to avoid too high concentrations of organic matter. If mixed with other effluents it can increase organic load up to 40%.
Water saving is possible by modifying the production cycle, with recycling the process water and by-products (blood, guts, etc.).
Recovery of water should not be excessive in volume and in time in order to avoid bacteriological problems.
Having to maintain a safety content and very high hygiene standard, it is unwise to design a system for the final recovery of water due to both the maintenance costs that those of embodiment.