Separate Sewers

Sewerage systems can be classified into combined sewerage (also known as conventional sewers) and separate sewerage. Combined sewerage carries both surface run-off and wastewater, while separate sewerage carries surface run-off and wastewater separately. Recent trends have been for the development of separate sewerage systems. By replacing the combined sewer systems with separate sewer systems, sewage and surface run-off can be managed in two separate systems. The main reason for this is that surface run-off is generally less polluted than wastewater, and that treatment of combined wastewater and surface run-off is difficult during heavy rainfalls, resulting in untreated overflows (commonly termed combined sewer overflow, UNEP and MURDOCH 2004). Controlling the surface run-off separately and avoiding combined sewer overflow,residents in low-lying areas in particular will avoid having their basements and ground floors flooded during extreme rain events. Should a rain event lead to flooding nonetheless, rainwater and not unsanitary sewage from kitchens and bathrooms will rise up into their basements.

Separation also eliminates the risk of sewage getting into the environment.Instead, it will be carried on to the treatment plants via a closed system, while storm water can be led to detention basins and watercourses (LADEN 2010). However, in practice there remains risk ingress of surface run-off into wastewater sewerage pipes, because of unsealed pipe joints, and unintentional or illegal connections of rainwater run-off. Conversely there may be unintentional or illegal wastewater connections to storm, water sewerage (UNEP & MURDOCH 2004).

Sewer separation has been used to eliminate combined sewer overflows for over 50 years. Today, separate sanitary sewer and storm water systems are more and more common. However, sewer separation is often said to be too disruptive, costly, and may result in an increase in pollutant loading to receiving waters as a result of the increased discharge of untreated surface run-off. Incomplete sewer separation efforts can also result in poor sanitary sewer system performance, including basement flooding and sanitary sewer overflows (TUIG et al. 2009).

Moreover, storm water drains in heavily urbanised areas may receive a variety of pollutants from rooftops, lorry loading bays, industrial sites and even from illegal connections to the surface water system. Therefore, publicity and inspections are needed to ensure that unexpected pollution does not arise from these sources (see also standards or restrictions). Pollutant loads discharged from urban storm water drainage systems not only vary depending on the urban structure, and the variety of materials entering the sewer network, but also on the local rainfall patterns and the processes of mixing and degradation that occur in the sewers. Hence, careful planning of the sewerage network is required to address these problems. Various modelling tools, such as rainfall modelling, sewer and sewage treatment modelling and river quality modelling, are now available to assist in this planning process (HELMER and HESPANHOL 1997).

The construction costs might be higher than for the combined sewer system because two separated networks are necessary. These must also be maintained and operated separately. Also to replace the combined system by a separated system is very costly.

(adapted from HELMER and HESPANHOL 1997)

Operation and maintenance (O&M) is an essential part of wastewater management and affects technology selection. Many wastewater treatment projects fail or perform poorly after construction because of inadequate O&M. On an annual basis, the O&M expenditures of treatment and sewage collection are typically in the same order of magnitude as the depreciation on the capital investment. Operation and maintenance requires:

• Careful exhaustive planning (specialised engineers).
• Qualified and trained staff devoted to its assignment.
• An extensive and operational system providing spare parts and O&M utilities.
• A maintenance and repair schedule, crew and facility.
• A management atmosphere that aims at ensuring a reliable service with a minimum of interruptions.
• A substantial annual budget that is uniquely devoted to O&M and service improvement.

Maintenance policy can be corrective, i.e. repair or action is undertaken when breakdown is noticed, but this leads to service interruption and hence dissatisfied customers. Ideally, maintenance is preventive, i.e. replacement of mechanical parts is carried out at the end of their expected lifetime. This allows optimal budgeting and maintenance schedules that have minimal impact on service quality. Clearly, O&M requirements are important factors when selecting a technology; process design should provide for optimal, but low cost, O&M.

Already the combined sewer system technology provides a high level of hygiene and comfort. A properly constructed separated system makes it even more secure, because the sewage is transported in a closed system directly to the treatment plant and cannot overflow into the environment. For general information about conventional sewers click here.