analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

from the clarified zone between the sludge and scum layers. The outlet is usually fitted with an
effluent screen and/or a tee-piece to retain larger solids that would otherwise be carried out in the
effluent to the soak-away, contributing to clogging and eventual system failure. Inspection ports and
manholes are provided in the tank cover to allow access for the periodic removal of tank contents,
including the accumulated scum and sludge.
Compartmentalised tanks such as that shown in Figure 2.6, or tanks placed in series are reported to
provide better suspended solids removal than single-compartment tanks alone, although results from
different studies vary.
Septic tanks are reported to remove 60 to 80% of non-soluble material in domestic wastewater. Solid
and colloidal material is hydrolysed and acidified, producing volatile fatty acids that are only partially
converted to CH4, and exit in the effluent stream. Biological oxygen demand (BOD) removal is
typically in the order of 30 to 50% for a septic tank operating at a 48 h retention time. Actual
performance of the septic tank will depend on the ambient temperature, operating hydraulic retention
time and presence of inert or micro-organism-inhibiting chemicals in the influent. Table 2.3 presents
septic tank effluent characteristics from a number of systems, before secondary treatment in a soakaway
or other system.
Soak-away systems provide a degree of wastewater treatment and dispersal through soil purification
processes and ground water recharge. The performance is dependent on the treatment efficiency of the
septic tank, the method of wastewater distribution and loading to the soil infiltrative surface, and the
properties of the vadose and saturated zones underlying the infiltrative surface. Considerable data on
the treatment efficiency of soak-away systems are available in the trade literature (USEPA, 2002).
High removal rates of BOD, suspended solids, faecal coliforms and surfactants have been observed
within a few metres of unsaturated, aerobic soil. Phosphorus and metals are removed through
adsorption, ion-exchange and precipitation reactions, although the retention capacity of the soil has a
limit that depends on specific soil properties including soil mineralogy, organic content, pH, redox
potential, and cation exchange capacity. Pathogen survival rates have been found to vary with a
number of factors including initial pathogen load, temperature, humidity and solar radiation.
Table 2.4 shows typical pathogen survival times in fresh water, sewage and unsaturated soil at 20 ºC
to 30 ºC (Feachem et al., 1983). Bacterial and protozoan pathogens have relatively short survival times
(less than a month), but viruses and helminth eggs (e.g. Ascaris spp.) can survive for considerably
longer periods.
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