Impacts of Stormwater and Groundwater Ingress on Municipal Sanitation Services

Impacts of Stormwater and Groundwater Ingress on Municipal Sanitation Services
Report No. 1386/1/05
March 2005

EXECUTIVE SUMMARY

South Africa has experienced rapid growth in water service provision, particularly in the last decades. In the past, standards have been sacrificed and many older and poor quality sewers are showing signs of leaking. Insufficient attention is paid to maintenance and rehabilitation resulting in overloading of sewers and Waste Water Treatment Works.

Sewer blockages and collapses take place due to penetration of roots, structural movement, opening of joints, corrosion, sedimentation and inadequate construction. Besides stoppages and collapses, stormwater inflows and ongoing groundwater infiltration (or so-called extraneous flows) can reduce the originally designated capacity of a sewer collection system and negatively affect operation of the entire waterborne sanitation system including the wastewater treatment component (WWTP). Increases in extraneous flows reduce the effectiveness of the biological process leading to higher pollution loads leaving the WWTP and disposing partially treated urban wastewater into adjacent river ecosystems.

Urban wastewater quantity and quality management issues and problems are now equally important for either centralized or decentralized disposal of urban wastewater return flows. Both quantity and quality of such water is crucial to the well-bring of other water users located downstream of the sources discharging wastewater and stormwater.

The urban water cycle is adversely affected by extraneous flows. If there is exfiltration, there can be groundwater pollution which also affects the catchment water balance. Water consumption is higher if plumbing leaks and more water is cycled on a macro scale. This increases the overall water supply cost as well as causing pollution. The linkage of water flow between water supply input point and treated effluent output point enables observation of the whole urban water cycle. The evaluation of return flows and consumptive use is a critical component in water resources development studies at various wastewater catchment levels. Stormwater inflows and groundwater infiltration into sewers have costly implications. Although in the past it has been the practice to allow spare sewer capacity, this can increase sewer costs by 10%-30%, and wastewater treatment by some 10%. The alternatives available to mitigate extraneous flows are:

Allow spare capacity in sewers and wastewater treatment plant (WWTP).
Lay pipes to high specifications to minimize infiltration/exfiltration
Refurbish existing deteriorated sewers
Manage the feeder area to minimize extraneous flows
Stormwater management (e.g. retention or detention holding dams at WWTP)

A questionnaire distributed nationally produced alarming data. Little attention is paid to inflows and infiltration and it is customary to allow for up to 50% of sewer capacity and also to design WWTP to cope with this flow. This project identified low awareness about I/I/E problems and remedial/rehabilitation techniques by most South African WSAs/WSPs. Due to the magnitude and complexities inherent to municipal waterborne sewer systems, WSAs/ WSPs cannot make an educated decision on developing a new or upgrading/rehabilitating existing systems without a mixture of field and modelled data.

One of the major conclusions in this project is that most WSAs/WSPs in South Africa resort to reactive maintenance, where problems are dealt with on a corrective basis as they arise. Consequently, municipal wastewater system maintenance budgets are commonly low and are based on the previous year's financial expenditure on clogging and collapses. Stoppages and clogging of sewers in South Africa per unit length of sewer are about ten times higher than the international average. In the meantime, the deterioration of municipal waterborne sewers continues to the point of failure and beyond.
In order to gauge general awareness about inflow/infiltration (I/I) problems in municipal sanitation systems in South Africa, a nation-wide e-mail survey was conducted under this research project. The survey generated new valuable information and verified several parameters for the development/enhancement of urban separated sewer systems.

It was established that most municipal sewer systems in South Africa have been in existence for 30 to 50 years and the aging process is taking its toll so that issues related to rehabilitation or replacement are becoming more important to the WSAa/WSPs. The type of materials used in the construction of sewer systems have also changed from clay and concrete to uPVC and AC piping, generating different problems.

The sample survey indicated that the most common causes of stormwater inflows and groundwater infiltration in the South African context are as follows:

Inadequate design of certain system components,
Illegal house down-pipe connections to the municipal sewers (all surveyed municipalities operate separate instead of combined sewers),
Open gullies serve primarily as sullage disposal (this is typically in most formal and informal townships),
Unsealed manholes primarily due to theft of the manhole covers,
Faulty pipe joints due to improper construction or deterioration,
Roots penetrating joints
Unwise man-made stormwater channelisation (e.g. road crossings and culverts) and unattended overgrown vegetation in natural channels, and
High groundwater table.

Next to the common causes generated by this survey, other factors were identified which can contribute locally to inflow/infiltration to sewers:

Undulating topography may lead to easy flooding due to marginal changes in stormwater flows,
Re-considered flood lines,
Swimming pools can be a contributing factor if additional stormwater or backwash water is linked directly to the sewers,
Ground movement due to removed mine dumps destroying continuity of sewers, and
Thunderstorms of short duration and higher intensities in various locations.

From the limited but representative sample of the nation-wide survey, it is concluded that the typical average sewer blockage rate is 3,3 blockages/km sewer pipe/p.a. This figure is more than double the average commonly quoted in the limited literature in South Africa of 1,2 blockages/km pipe/year and far supercedes international averages. This aspect will influence the calculation of costs if trenchless technology is adopted in sewer maintenance programmes. The most common materials used by the municipalities are identified as uPVC and AC piping, in pipe diameters ranging from 100 to 1000 mm. A surprising aspect of municipal sewer systems is the large number of pumping stations built in some of the existing systems which give rise to a number of problems.

An area in the Ekurhuleni Metropolitan Municipality (namely Boksburg urban area) was assessed as a case study. Next to residential and recreation grounds, it abuts an industrial area and sewers are built through waterlogged ground. It was found that stormwater inflows amounted to up to 40% of sewer capacity and groundwater infiltration amounted to 15% of capacity. Leaking household and faulty plumbing contributed a comparable amount. Field investigation methodologies and inflow/infiltration monitoring results compiled by others in the urban areas of Gauteng province and elsewhere in South Africa were evaluated and relevant findings were combined into this report. Typical values of key sewer flow components are as follows:

Residential wastewater outflows range between 0,01 and 1,20 l/min/household
Water leakages into municipal waterborne sewers range between 0,06 and 0,20 l/min/household
Groundwater infiltration into municipal sewers ranges between 0,01 and 0,50 l/min/m-dia/m-pipe for all types of sewer materials

From technical reports available on the subject of inflow/infiltration in Boksburg and Benoni, the following were identified:

Stormwater and surface inflows account for dramatic peak flows (up to 3 times the AADWF) experienced, particularly in the Boksburg Outfall. The source of the inflows can be attributed predominantly to household stormwater being directed into the sewer system through gulleys, and to a lesser extent, due to missing or damaged manhole covers.
Ground water infiltration produces a steady base flow in the sewers, which increases treatment costs and reduces the operating capacity in downstream sewers. It appears that only in severe cases where the extent of groundwater infiltration may cause structural collapse or substantial reduction in the capacity that pipe replacement or repair makes financial and practical sense.
Both stormwater inflow and groundwater infiltration are continuing to increase due to reactive maintenance instead of planned preventative maintenance and planned rehabilitation programmes.

The key conclusion this project identified was low awareness about I/I problems and remedial/rehabilitation techniques by most South African WSAs/WSPs. Due to the magnitude and complexities inherent to municipal waterborne sewer systems, only a few WSAs/WSPs can make an educated decision on developing a new or upgrading/rehabilitating an existing system (or its key components). They lack mainly field and modeled data, particularly on inflow/infiltration/ exfiltration events and their consequences.

Guidelines concerning construction and rehabilitation of sewers were prepared as part of the contract. Methods of evaluating the problem and remedying the situation were listed. It is recommended to consider that groundwater infiltration exceeding 0,10 l/min/m-dia/m-pipe is excessive for all sewer pipe materials. Another major observation from this project is that the maintenance strategy of most WSA/WSPs in South Africa is essentially reactive maintenance, where problems are dealt with on a corrective basis as they arise. Consequently, municipal wastewater system maintenance budgets are commonly low and are based on the previous year's financial expenditure mainly from clogging and collapsing sewers. It has been established from a survey that stoppages and clogging of sewers in South Africa per are about ten times higher than the international average, averaging to 3,3 blockages/km/sewer pipe/per annum.

It was also established by this project that the costs associated with maintaining or expanding existing and/or developing new urban wastewater infrastructure appear to be large, but well invested if allocated on a regular basis. Because there is not yet enough pressure applied from the wastewater services end-users to municipal managers about the economics of alternative solutions, conventional methods prevail and benefits are not highlighted in the cost analysis. Water infiltration in sewer pipelines is common and should be included in the peak design flow. A norm of 15% of the dry weather flow allowance for extraneous flows is a generally acceptable standard. Flows exceeding this norm will result in pipe capacity problems and an unnecessary increase in sewer discharge volumes and treatment costs. A reduction in infiltration/inflow rates will not only save on sewerage treatment costs, but may defer capital expenditure for the upsizing of collection sewer pipelines and wastewater treatment plant. The decision to solve or ignore an infiltration problem should therefore be based on a benefit-cost analysis.

Due to the magnitude and complexity of the attention required to research, design, construction and management of wastewater sanitation systems, all relevant stakeholders must share responsibility for development and management of these systems. The application of new local and international technologies must be promoted by the WSAs and WSPs through adopting the Guidelines in capacity building programmes.

To sustain a reliable municipal wastewater infrastructure and required service to customers, new and improved solutions to existing and emerging problems will have to be researched. Spheres for further research relate to flow monitoring, assessment of structural integrity, operation and maintenance programmes, and new methods in rehabilitation of sewers.