282 Organic waste recycling: technology and management
Figure 6.10 Mean individual fish weight (g) at monthly intervals in the septage-fed fish
pond system. Each point is a mean of three ponds. Organic loading = 150 kg COD/ (ha-
day). Stocking densities (SD) in number of fish/m^2 (Edwards et al. 1984)
The relationships between H 2 S, HS- and S2- at various pH levels are shown in
Figure 6.12. At pH values of 8 and above, most of the reduced sulfur exists in
solution as HS- and S2- ions, and little amount of un-ionized H 2 S, the
malodorous gas, is present. However, the formation of un-ionized H 2 S is
significant at pH levels below 8, which can cause detrimental effects to fish
production and producing the malodorous problems.
It is interesting to note that, although an increase in water pH to above 8 can
avoid H 2 S formation, this pH condition would enhance the formation of the un-
ionized NH 3 compounds (see Figure 6.9), which is toxic to fish. Therefore
organic loadings to fish ponds have to be properly controlled to avoid the
occurrence of anaerobic conditions and DO depletion. Sludge deposits should be
Time (months)Mean Fish Weight, g