CONTINUOUS FARM SCALE AERATION PLANT FOR REDUCING OFFENSIVE ODORS FROM PIGGERY SLURRY - CONTROL AND OPTIMIZATION OF THE PROCESS

The concept of optimizing a continuous aeration process for pig slurry by varying the treatment time to reflect the solids content of the feed, was demonstrated. The target quality of the treated slurry was defined to be that which would store for 25 d before the return of offensive odours (as indicated by a total volatile fatty acid concentration exceeding 0.23 kg/m(3)). Correlations developed in earlier studies were used to link the solids content of the raw slurry to its chemical oxygen demand (COD) value and to the length of treatment required. Methods for direct measurement of solids content were found to be both expensive and unreliable so a novel approach was developed based on the relationship of separator performance to the total solids concentration of the raw slurry. Using this information, the required treatment time was periodically revised to reflect changes in the feed slurry, and thus ensure that the target quality was maintained. The viability of this strategy was tested by a continuous computer-controlled trial lasting 22 d. Measurements of the VFA of treated samples after 25 d storage indicated that control was working if allowance is made for correctable errors. Potential savings from the optimization strategy will depend on the variability of the raw slurry concentration. On the basis that all slurry treated should meet the 25 d storage criterion, a fixed treatment time would need to be set to treat the thickest slurry expected. For the trial carried out, this fixed treatment time would have been 4 d. However, with the control system in operation, treatment lasting 2 d sufficed for half of the trial period. The saving in cost implied for this case is around 25%.