Slaughterhouse in Ireland, part of an international group, found themselves unable to comply with their effluent discharge consent levels following the introduction of legislation adopting EU Environmental Directives.
The slaughterhouse, on average, processes 200 sheep per day. The plant produces an effluent flow of approximately 200 m3/day and this, after treatment, is discharged into a river.
None of these targets were being achieved when Cleveland Biotech (CBL) were brought in to advise the company on how to meet their consent levels. The ammonia and nitrate discharges were causing particular concern as these were running at 5 to 6 times above consent levels.
From a review of the treatment plant it was obvious that the plant was not being operated in a manner to optimise the microbiological potential. The first action was to modify the plant so that the most effective treatment pathway was selected. Testing showed that there were no inhibitors to nitrification in the waste flow but the BOD passing from the balance tank to the aeration tank was averaging 1000-1200mg/l. With this level of carbonaceous matter present little nitrification was occurring in the aeration basin. This was accentuated by the low levels of DO in the aeration basin.
It was decided that the problem could be tackled most economically in two phases:
Phase 1: to adapt the existing plant so as to increase its biological performance so that it could achieve compliance with the licence for BOD, SS and NH4+.
Phase 2: once the final nitrate level was established to design and add to the plant an anoxic zone where denitrification could occur and compliance with the nitrate licence be achieved.
The alterations proposed to the plant were:
- To raise the DO level in the aeration tank to an effective level by substituting the 6kw aerator with a 22kw submersible aerator.
- To use the 104m3 balancing tank as a treatment tank in order to reduce the BOD at the influent to the aeration tank to a level of 200mg/l. To achieve this the tank needed to be aerated and high levels of organic digesting bacteria added to obtain the necessary BOD reduction within the available retention time. CBL proposed the installation of the existing 6kw aerator in the balance tank and, as a means of supplying economically the high numbers of bacteria required, the installation of a CBL ABT 1 Reactor which delivers a high volume of bacteria on site from a small inoculum dose. This reactor would supply bacteria on a daily basis into the balancing tank.
CBL’s proposals were accepted in April. The reactor was installed and commissioned in early May. The alterations to the aeration systems were however delayed and not completed until the third week of May. The results now being achieved are very satisfactory. The factory is now comfortably meeting its consent levels for BOD, SS and NH4+ as is shown by the attached graphs. As a result of the degradation of the ammonia, as was expected, nitrate levels have risen. The level of nitrate which requires reduction is now known and Phase 2 of the project is in the design stage and is expected to be commissioned before the end of the year.
By the use of biomass engineering the factory has achieved its consent levels at minimum cost and with minimum interruption to its operations. It has also avoided the substantial fines and possible closure which would have resulted if it effluent discharges could not have been brought within consent levels.