There are a few items to consider for this situation. Average ph of effluent is 6.7. If the volume of waste is in advanced decomposition the ph will become more acid. (carbon + water = Carbonic acid). This is possible but I do not believe it would happen overnight. The major problem with developing countries is the illegal dumping of industrial waste. This would be the first place to look. As a side note rain is known to be slightly acidic. But when your treatment facility is near heavy polluting smoke stacks the rain again will cause the carbon to wash from the air and make carbonic acid. To resolve all of these negative situations you may want to consider microbial bioremediation. Using natures microbial balance of RNA and DNA microbes will do the trick. All waste reduced to elemental state. All toxic material reduced to their non toxic/nutritional form. Good luck!