In the semi-arid Yemen Republic wastewater treatment has high priority to control the spreading of communicable diseases and to make the effluent fit for reuse in agriculture. Experience with existing waste stabilisation ponds has in many cases been unfavourable because of poor BOD removal and odour problems. The raw domestic sewage at the Sana'a waste stabilisation ponds is very strong in terms of BOD, COD, TSS and sulfate, and its ammonia levels are excessively high (150-200 mg N-[NH3 + NH4+]L). The combination of high temperature, high organic loading rates and the presence of sulfides, due to conversion of sulfate in the anaerobic ponds, appears to favour development of anoxygenic photo-heterotrophic purple non-sulfur bacteria Rhodopseudomonas spp. in facultative ponds. Some species can oxidise sulfides to sulfate using light; they lend the water its typical reddish-pink colour and cause high turbidity which may disfavour algal growth. Also, in large tracts of the ponds the high ammonia levels suppress algal growth, as demonstrated in lab experiments. Consequently, low DO and poor C-metabolisation prevail in the ponds. Low DO in turn again creates conditions stimulating microbial sulfate reduction. Field data from other facultative ponds in Yemen suggests that a minimum retention time in the facultative ponds of 20-25 days is necessary to provide ecological conditions allowing algae to outcompete purple bacteria.

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