Table 1.

Overview of various secondary treatment options available.

Treatment	Design criteria	Effluent quality	Advantages	Disadvantages	Ref.
WASTE STABILISATION PONDS
Anaerobic ponds	2–5 m deep, pH usually below 6.5; less surface area; covered either by gravel, plants, steel, and plastic. Loaded at high rates to prevent inlet of any oxygen	BOD Removal of 60%–85%	Low cost, little excess sludge produced, Small pond volume needed; Low nutrient requirements; Low operating costs; no electricity required; Methane by-product	Requires more land; Long start-up period; Post treatment always required, can produce an unpleasant odour; Requires sludge removal more often; Operates optimally at warmer temperatures (>25 °C)	[10,19]
Facultative ponds	Shallow—1–3 m deep; Length to breadth ratio should be a minimum of 2:1; lined with compact clay (minimum thickness 0.3 m) or polyethylene; formation of two layers—aerobic at surface and anaerobic at bottom	BOD removal of 70%–85%	Efficient BOD reduction; Nutrient reduction by aerobic and anaerobic bacterial processes as well as by surrounding plants; Natural aeration of the upper layer via movement of air; Low energy consumption	Significant space requirements; Efficiency is strongly affected by environmental factors; continuous maintenance required	[10]
Maturation ponds (polishing ponds)	Shallow—0.9–1 m deep; allows for light penetration; completely aerobic; high pH and high concentration of dissolved oxygen due to algal activity; little biological stratification; size and number depends on required effluent pathogen concentration	Little BOD removal because most has been removed in previous stages	Removes excess nutrients and pathogens such as faecal coliforms	Small BOD removal; additional costs; additional land requirements	[10]
SUSPENDED GROWTH SYSTEMS
Activated sludge	oxygen supplied for initial sludge decomposition and provide agitation to promote flocculation; 85% sludge removed whilst 15% recirculated	BOD removal of 90%–98%	Production of high quality effluent; reasonable operational and maintenance costs	High capital costs; high energy consumption; regular monitoring required; back washing needed	[20]
Batch reactor	Equalization, biological treatment and secondary clarification are performed in a single reactor vessel using a timed control sequence; aeration may be provided by bubble diffusers/floating aerators	BOD removal of 89%–98%	Initial capital cost savings; all processes carried out in a single reactor vessel; timed cycles; requires limited land; equalization of processes	Higher level of sophistication and maintenance required as timing must be controlled; may discharge settled or floating sludge; clogging of aeration devices; requires oversized outfalls as effluent discharge is timed	[21,22]
SUSPENDED GROWTH SYSTEMS
Aerated lagoons	Should be lined with clay or some natural source, 1.8–6 m depth, 10–30 day retention time, oxygen supplied by additional mechanical means	BOD removal of up to 95%	Low cost, low maintenance and energy requirements, can be well integrated into surrounding landscapes, reliable treatment even at high loads	Nutrient removal is less efficient due to short retention times	[23,24]
FIXED FILM SYSTEMS
Conventional biofilters (trickling filters)	Bed with supportive media such as stones, plastic, wood; 0.9–2.4 m deep; oxygen supplied via natural flow of air	BOD Removal of between 80%–90%	Low land requirement Moderate level of skill required for operation and maintenance Suitable for small to medium communities	Accumulation of excess biomass will affect performance; high level of clogging thus regular backwashing is required; if suddenly shut down–anaerobic conditions result in reduced effluent quality; odour and snail problems	[25,26]
Rotating biological contactors			High contact time; high effluent quality; resistant to shock hydraulic or organic loading; short contact periods; large active surface area; silent; low sludge production; easy transfer of oxygen from air	Continuous power supply required; oxygen may be a limiting substrate	[27]
Biological aerated filters	Consists of a reactor container, media for supporting biofilm growth, influent distribution and effluent collection system;Optimal conditions—pH 6.5–7.5 with mixing; Media should be chemically stable, high surface area and low weight e.g., sunken clay, floating polystyrene beads	High nutrient removal (80%–100%)	Environmental factors such as pH, temperature will aid microbial growth; high removal efficiencies; can combine ammonia oxidation and solids removal in a single unit	Media may become clogged due to biomass growth and accumulation—may create resistance to air and flow of liquid; regular back washing is required to remove excess biomass and particles	[28,29]