Water and Wastewater Engineering

Learning Objectives

Sewage or municipal wastewater includes human and household wastes, street washing, wastewater from industrial and commercial activities in the municipal area, groundwater and stormwater. There are two major types of sewage collection systems: dry or conservancy systems in unsewered areas and wet carriage systems in sewered areas (Duggal 2007).

The dry or conservancy system is common in rural and semi-urban areas where there are no sewer systems. Night soil (feces and urine, i.e., human excreta) is collected in latrines, privies, septic tanks, cesspits, or twin-pit latrine systems. Anaerobic conditions develop and reduce the solid concentrations in the sewage in these systems, which are removed manually or mechanically at relatively long intervals of 1–5 years.

Wet carriage systems are common in urban centers where sewerage systems have been provided. Wastewater is collected from individual buildings (toilets, sinks, bathrooms, any other water using facilities) and taken for treatment to a sewage treatment plant. The wet carriage system is popular in urban centers all over the world since it does not require manual transport of excreta, it results in better hygiene and reduced risk of waterborne diseases, has less odor and is aesthetically better than the dry systems. The only disadvantages are the high capital and maintenance costs, and excessive water consumption. Only wet carriage systems are addressed in this textbook.

Conventional and advanced wastewater treatment schemes were discussed in Chapter 7, Section 7.3. After treatment, municipal wastewater can be reused or disposed in various ways and are discussed in this chapter.

MUNICIPAL WASTEWATER COLLECTION SYSTEMS

Wastewater collection systems are generally designed as combined sewer overflow (CSO) especially in old cities with sewerage systems where wet and dry weather flows are collected and treated together. However, a relatively recent trend which started in the twentieth century (Mannina and Viviani 2009) is to design separate sewage collection systems for dry and wet weather flows, respectively. Separate collection systems have two drainage networks, one for stormwater flows and the other for sewage flows, which result in higher capital costs. However, cost-benefit analysis and comparison of the two systems remains a difficult and debatable issue with no clear answers.