MLBA 2 : April / May 2008
Treatment of meat processing wastewater
A typical large meat processing plant in New Zealand produces a wastewater load equivalent to that from a city with a population of about 400,000 people.
Appropriate and effective treatment of the wastewater and disposal of the treated effluent is therefore of major importance.
Wastewater from meat processing typically has a very high strength and high nitrogen content so that special methods of treatment are required in order to reduce the strength and also to reduce the nutrient and bacteria concentrations to levels that allow discharge of the effluent to the environment.
Wastewater composition
The strength and composition of wastewater from meat processing depend on the processes carried out at the individual meat processing plant (such as killing, cutting up and packing, rendering and hides processing) and the mix of animals processed.
Wastewater from meat processing is characterised by high strength (COD and BOD), high nitrogen content, high fat and grease content.
It also usually contains very high concentrations of faecal coliform bacteria due to the inclusion of stockyard manure as well as gut contents.
The strength of meat processing wastewater is markedly affected by the volume of water used in processing, with older meat processing plants using as much as 500L/lamb equivalent and modern plants using as little as 150L/lamb equivalent.
Consequently, the strength and composition of meat processing wastewater is not the same for all plants.
The typical composition of wastewater from a large, older meat processing plant with a slaughterhouse, rendering operations and water usage of about 300L/lamb equivalent is shown in Table 1 below:
Historical methods of treating or disposing wastewater
Historically, meat processing wastewater has often not been treated at all but simply disposed of by irrigation onto land, discharge into a municipal sewerage system, or in some cases, discharged to a river or the ocean.
The most common methods of treatment of meat processing wastewater have been by dissolved air flotation (DAF) and/or anaerobic lagoons.
DAF is effective at reducing the suspended solids and fat content of meat processing wastewater and can also remove nitrogen in the form of protein if acid is used to lower the pH within the DAF to below the iso-electric point at which protein comes out of solution. Neutralisation of the DAF effluent is then required. Anaerobic lagoons work partly through anaerobic (biological) breakdown of the BOD in the wastewater and partly by settlement of solids to the bottom of the un-mixed lagoon.
The accumulation of sludge in an anaerobic lagoon ultimately requires a de-sludging operation to maintain the active volume in the lagoon.
Anaerobic lagoons are popular because they are capable of providing significant treatment at low operating cost because there is no need for power for aeration as would be required for aerobic processes. However, an aerobic process is necessary where a high effluent quality is required.
The recommended design and operation of anaerobic lagoons for treatment of meat processing wastewater is described in MIRINZ Bulletin No.28.
Anaerobic lagoons have usually been constructed by simply excavating an earth basin with a depth of about 4m and allowing the fat and solids in the wastewater to form a thick scum on the surface that can contain odour and even treat the odour to some extent when soil and grass grow on the cover.
DAF cannot be used prior to an anaerobic lagoon if a cover is to be developed because the DAF removes that fat and solids necessary to create the cover.
Once established, the cover layer on an anaerobic lagoon can look just like pasture, which creates a potential hazard for animals if the lagoon is not fenced off. Where meat processing wastewater is discharged to a municipal sewerage system it has often caused problems with odour generation or high operating costs due to the high strength of the wastewater.
This has especially been the case where the municipal wastewater treatment facility is an oxidation pond system with little capacity to respond to peak loads.
Mechanical aeration is required to expand the capacity of ponds to treat wastewater with a high content of wastewater from a meat processing plant.
The current situation
There are still meat processing plants that carry out little or no treatment and discharge untreated wastewater to land, a river, or to the ocean.
Other meat processing plants discharge wastewater to municipal sewerage systems and will probably continue to do so where the trade waste charges are lower than the costs of constructing and operating a separate treatment and disposal scheme for the plant.
It is unlikely that the meat processing plants that currently discharge virtually untreated wastewater to a river or to the ocean will be allowed to continue these practices.
Governments, responding to public expectations and environmental concerns, are forcing such plants to provide treatment of their wastewater to minimise adverse environmental effects.
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