Combined Aerobic and Electrolytic Treatment of Cattle Slurry

Abstract

Handling and use of raw cattle slurry can cause environmental and health problems. These may be solved by improving its physical, chemical and biological characteristics. As the slurry usually needs to be stored for long periods before spreading, methods for improving its quality, combined with storage, are considered in this paper. A total volume of 60 m 3 of cattle slurry was aerated in a 10 m 3 reactor and stored for 45 weeks in two covered concrete tanks, each of 30 m 3 capacity. The slurry in one tank was exposed to an electrolytic treatment, while in the other, no further treatment was given. In a third tank of similar design, raw slurry was stored without any form of treatment. The content of thermotolerant coliform bacteria (TCB) was reduced from 10 4 TCB/g of slurry to below 10 2 TCB/g during the 7 d treatment in the aerobic reactor. No new TCB growth was detected during the following 45 week storage period. The electrolytic treatment seemed to ensure an effective pathogen kill. Also raw slurry achieved <10 2 TCB/g after 23 weeks of storage. Aeration reduced the odour considerably compared with raw slurry. Additional electrolytic treatment had a strong and immediate effect, and almost no odour was noticeable. Aeration also reduced hydrogen sulphide emission from about 2000 p.p.m. in the raw slurry to about 300 p.p.m. Additional electrolytic treatment immediately reduced this level to about 150 p.p.m. and further to zero after 10 weeks. Only 0·13% of total-N was lost from the aerobic reactor, and this amount was absorbed by a biofilter. After 45 weeks storage the amount of ammonium-N was equal in raw, aerated, and combined aerated and electrolysed slurries. During the electrolytic treatment the Cu content in the slurry increased from 20 mg/kg TS to about 300 during the first ten weeks, by which time all the desired treatment effects had been obtained. Long-term storage of raw cattle slurry did not solve the odour and hydrogen sulphide problems, but pathogen kill was effective. Aerobic treatment in a reactor produced a slurry with practically no TCB, low odour level and low hydrogen sulphide emission, without losing nitrogen. The aerated slurry was suitable for long-term storage. Additional electrolytic treatment of the aerated cattle slurry further reduced the pathogen, odour and hydrogen sulphide concentrations. By combining aerobic and electrolytic treatment it was possible to achieve full control of pathogen bacteria, odour and hydrogen sulphide.