This system is a combination of an open digester tank and a series of ponding systems (Fig. 7). The anaerobic digestion is carried out in the digester, then in the facultative anaerobic and algae ponds. It has been shown that by using an open digester, a better reduction of BOD can be achieved in a shorter time. Digesters are constructed of mild steel at various volumetric capacities ranging from 600 up to 3600 m3. The treatment of treated POME from the digester
will start at the facultative ponds, followed by the algae ponds. A description of the ponding systems is outlined in the previous section "Pretreatment."
The HRT of the digester is only 20-25 days and has a higher organic loading of 0.81.0 BOD kg/m3 ■ day compared to anaerobic ponds. With minimal financial input from the operators, no mechanical mixing equipment is installed in the digesters. Using the same principle as anaerobic ponds, mixing of POME is achieved via bubbling of biogas. Occasionally, the mixing is also achieved when the digester is being recharged with fresh
POME. The treated POME is then overflowed into the ponding system for further treatment.
Although the digester system has been proven to be superior to anaerobic ponds, it also has similar problems of scum formation and solid sludge accumulation. Another serious problem is
the corrosion of the steel structures due to long exposure to hydrogen sulfide. Incidents such as burst and collapsed digesters have been recorded. Accumulated solids could be easily removed using the sludge pipe located at the bottom of the digester. The dewatered and dried sludge can then be disposed for land application.
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