PURPLE NONSULFUR BACTERIA AND THEIR INFLUENCE ON WASTE STABILIZATION POND PERFORMANCE IN THE YEMEN-REPUBLIC

In the semi-arid Yemen Republic wastewater treatment has high priority to control the spreading of communicable diseases and to make the eff luent fit for reuse in agriculture. Experience with existing waste sta bilisation ponds has in many cases been unfavourable because of poor B OD removal and odour problems. The raw domestic sewage at the Sana'a w aste stabilisation ponds is very strong in terms of BOD, GOD, TSS and sulfate, and its ammonia levels are excessively high (150-200 mg N-[NH 3 + NH4+]/L), The combination of high temperature, high organic loadin g 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 fac ultative ponds, Some species can oxidise sulfides to sulfate using lig ht; 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 demonstrat ed in lab experiments, Consequently, low DO and poor C-metabolisation prevail in the ponds. Low DO in turn again creates conditions stimulat ing microbial sulfate reduction. Field data from other facultative pon ds in Yemen suggests that a minimum retention time in the facultative ponds of 20-25 days is necessary to provide ecological conditions allo wing algae to outcompete purple bacteria,