Basically wastewater generated from slaughterhouses will originate from two different sources. The first will be from the temporary holding pen and the other will be from the slaughtering activities. The strength of the wastewater will largely depend on the number of animals handled each day but more important is how these activities are being controlled and carried out. This is especially true for those slaughter process whereby large animals such as goats, cattle and pigs will generate huge amount of blood which can create strong wastewater stream if those are not properly collected and recovered back. There are actually two treatment options; one is to directly discharge it to the sewer and then diverted the wastewater to municipal treatment plant but this is often not the case as tightened rules by governmental bodies has made it difficult while another option to set up an on-site treatment facilities.
Slaughtering activities that generate the bulk of the wastewater normally comes from the regularly floor washing that will carry with it blood, bits of carcasses and that includes animal waste as well. Analyzing and studying the characteristics of the wastewater suggest that most of the compounds are highly biodegradable (indicated by the ratio of BOD:COD about 2:1) with relatively moderate amount of suspended solid materials present (about 1000ppm). A high percentage of the BOD in the water are actually contributed by the blood washed away and usually in order for the treatment facilities to be able to function well would require that the collected blood be handled separately or even better recovered. Therefore in order to achieve this, usually the best approach to adopt is to create a separate waste collection stream for individual area, much like the function of individual collection system. This way, sanitary wastewater can go directly to sewers while stockpen waste if possible, to avoid the use of water to wash but instead collected as solid material and leaving only the wastewater coming from the slaughtering activities to be diverted to the on-site plant.
Generally the same treatment process route are used and applied for all slaughterhouses whereby the first step would involve coarse screening so that hair and feathers are removed. This is important in order to prevent accumulation of the materials that will jam up mechanical equipment and usually for more efficient separation of solid particles; fine screens or traps are installed just before the pumps so that these will not interfere with the process later part. Wastewater that is sent for further treatment will often contain balances of oils and fats together with grease, suspended solids and of course the blood which will largely contribute towards your BOD value. Usually before it undergoes the aerobic treatment step using biological degradation process, the stream will have to be first diverted to the DAF unit so that the blood plus oil and grease constituents are reduced. If these are controlled well, expect at least 20 to 30% reduction of the original BOD value.
Usually the figure given here is rather low but the main contributing factor on why the system is operating below the normal design capacity is largely due to the fact that most slaughterhouses are not running 24 hours. If this is the case, it will mean that the DAF system will have to be shut down and then restarted again when wastewater again becomes available and thus a steady-state of operation can never be achieved. One of the solutions to handle the problem is to construct a holding tank that will act as a buffer storage tank so that even when slaughtering activities are stopped, there will still be enough wastewater to supply for the operation of DAF system.
For the biological treatment process usually a number of unit processes are chosen that include the activated sludge process, sequencing batch reactor (SBR) and also the oxidation ditch. At outskirts area outside of large cities whereby land is readily available, use of lagoons can be considered as a good treatment option as well. Usually the first stage involve anaerobic process and a well-managed typical setup can contribute towards at least reduction of about 80% of the BOD value before the wastewater is sent to aerated lagoons where further BOD reduction takes place. There are different types of aerator unit which can be selected to improve the process and the decision to select which type of aeration system is purely based on cost allocation and fund availability. Constructing a rendering plant to recover the blood and suspended solids will also help and usually it is built adjacent to the slaughterhouse. This would greatly contribute towards reducing the biological load to the incoming wastewater stream and this would help the downstream process control becomes easier.