AEROBIC BIOLOGICAL-TREATMENT
SYSTEMS
Developments in wastewater ponds
In most countries, ponds remain the main form of
aerobic biological treatment for the removal of
COD from slaughterhouse wastewater, although a
wider variety of secondary biological systems has
also been used to some extent, including trickling
filters and activated-sludge systems. There have been
few exciting developments in the use of classical
pond types (facultative; aerated) to treat slaughterhouse
wastewater in the last decade (Sachon, 1984).
Some slaughterhouses treat hide-curing wastewater
in evaporative ponds. Tanji et al. (1992) studied
Californian evaporation ponds and found thatheavy-metal concentrations in salt evaporates were
well below hazardous levels and that, for agricultural
drainage liquids, the main salts crystallised were
relatively non-toxic sulphate, chloride and carbonate
salts of sodium, magnesium and calcium. In contrast,
it was found that concentrations of toxic heavy metals
exceeded hazardous levels in the concentrated
liquor of ponds, especially when conditions of
almost complete evaporation were approached. Care
is needed in the disposal of such liquid.
Three new concepts for ponds have emerged in
the last decade, but none have been applied to
slaughterhouse wastewater. High-rate algal ponds
involve an initial pond in which incoming wastewater
enters a deep anaerobic pond section, after which it
flows into an aerobic section. An aerobic environment
is maintained by recirculation of oxygen-rich
water from a second pond containing very high algal
concentrations. Israeli experience (domestic wastewater)
with these systems has been positive (Shelef
& Azov, 1987