Out of sight, out of mind? 

By Sue Matthews, Overstrand Estuary Management Coordinator 

Concerns about sewage pollution in the Klein River estuary date back at least 25 years. As far back as 1986, the local municipality initiated a monthly survey of faecal coliforms ‘in view of the possibility of seepage from septic tanks along the northern shore’. 

More recently, a 2½ year study that ended in January 2009 revealed high levels of faecal contamination at certain times of year, including the peak summer season. Fortunately, most sites complied with standards outlined in the water quality Guidelines for Recreational Use. 

However, sewage pollution is not only a potential threat to human health, but to the ecological health of the estuary too. Sewage contains high levels of nitrogen and phosphorus, which can result in nutrient enrichment (eutrophication) of the water column, leading to algal blooms that in turn cause oxygen depletion as they die and decay. 

The Stanford wastewater treatment works is generally compliant with the national standards it is subject to – although glitches do occur periodically – and its performance is closely monitored. The municipality takes monthly samples of its final effluent and sends them to an independent laboratory for analysis, while the Department of Water Affairs has monitoring stations above and below the discharge point in the river. In an effort to reduce nitrogen and phosphorus in the final effluent even further, an artificial reedbed has recently been created to serve as a ‘polishing’ stage before discharge into the river. 

However, the same level of control is not applied to septic and conservancy tanks, which may have a significant cumulative impact in terms of pollution of the estuary. Tanks older than 20 years – typically built with concrete or brick – are prone to crumbling and cracking, but it is also common practice for builders to knock a hole in newly constructed tanks once the building inspector has signed them off, to reduce the frequency of ‘honeysucker’ call-outs. If tanks are not watertight, they not only leak into the surrounding soil and may contaminate groundwater, but are also flooded by the seasonally high water table, which in the case of septic tanks results in liquid overflowing into the soakaway before it has been adequately treated. 

The Overstrand Municipality no longer allows new septic tanks with soakaways to be built – instead insisting on conservancy tanks that must be periodically emptied if there is no waterborne sewerage system in place. But there are still many older septic tanks along the estuary’s shores, some of which have soakaways within 30 m of the water’s edge! This is especially problematic given that the northern shore is essentially a giant seepage area below the Kleinriviersberg mountain range, with both surface runoff and groundwater flow directed towards the estuary. Ideally, any septic tank systems within 100 m of the estuary’s highest water level and/or the 5 m contour should be converted to conservancy tanks as soon as possible.

Studies have shown that even well-functioning septic tanks and soakaways (also called a drainage field or soil absorption system) have an impact on nearby waterbodies. While phosphorus is easily absorbed by the soil in the vicinity of the soakaway, nitrogen in the form of nitrate is highly mobile, moving with groundwater to form a plume extending more than 100 m.  

Septic tanks that have not been pumped for years are unlikely to be functioning effectively, even if problems such as backed-up loos are not experienced. This is because solids will have built up too much to allow adequate treatment time in the tank. The process relies on anaerobic bacteria to ‘digest’ sewage into three components: solids – which settle out to form a sludge layer on the bottom – liquid effluent, and a greasy scum layer on top of it, consisting of oils and fats that cannot be broken down but float on the surface. The digestion process takes 24-48 hours, so if the sludge layer gets too deep there is less space for the liquid, and it flows out to the soakaway before the bacteria have done their job properly. The soakaway then gets clogged to the point that it cannot perform its task – the aerobic phase in which soil bacteria further break down the effluent and convert ammonium to nitrate.

While there are various biological and chemical additives on the market that are touted as a cure-all for septic tank systems – claiming to stimulate bacterial activity, improve settling, break up scum or restore permeability in the soakaway soil – their use is contentious, and many states in the USA have banned their use as they are believed to do more harm than good. For example, the scum layer helps trap fat and grease, and breaking it up just causes more clogging of the soakaway. Additives in the soakaway may mean that pollutants infiltrate through the soil before being treated, contaminating the groundwater. 

There really is no substitute for regular maintenance, which involves inspection by an experienced professional every two to three years, and pumping when the sludge layer reaches a third of total depth. Typically this would be every three to five years, but would depend on the size of tank, the number of people generating waste and - in the case of holiday homes - the occupancy rate. 

A proper inspection – NEVER try this yourself, as the fumes from septic tanks are potentially lethal – would include checking internal baffles and the effluent filter if there is one, measuring scum and sludge layers, checking for sewage leaks or wastewater back-up, inspecting plumbing and mechanical parts, and checking the soakaway for sogginess or ponding, in which case a drainfield inspection involving minor excavation and soil permeability tests might be recommended. 

The system also needs to be handled with care on an ongoing basis. Apart from the obvious things - like not flushing cotton swabs and other ‘foreign objects’ down the loo - avoid pouring fats, grease and coffee grounds down the sink, use mild bathroom cleaners or baking soda rather than harsh anti-bacterial chemicals, and conserve water as much as possible to avoid overloading the system. However, all household wastewater should be directed through the septic tank - the practice of piping greywater into separate soakaways is not encouraged nowadays, as this water may be heavily polluted. 

Also make sure you know the location of your septic tank and soakaway. Put an easily moveable marker such as a birdbath over the tank lid to make it easy to find, and a barrier of some kind round the soakaway if there’s a risk of anybody driving over it, since compacting the soil will restrict oxygen levels. Divert surface water, including drainage from gutters and irrigation sprinklers, away from the soakaway, and don’t build or plant anything other than grass on top of it.  

Above all, change that ‘out of sight, out of mind’ attitude when you flush the loo, and help protect the Klein River and its estuary by accepting responsibility for your sewage effluent.   

For more information: http://www.nesc.wvu.edu/subpages/septic.cfm