Separate sewers

Provisions should be included for infiltration in the case of separate sewers as well as storm flows in the case of combined sewers. 

Sanitary sewer network sanitary sewers—often identified as separate sewers — are developed to collect and transport wastewater from residential areas, commercial areas and industries. The wastewater transported in these sewers typically has a relatively high concentration of biodegradable organic matter and is therefore biologically active. Wastewater in these sewers is, from a process point of view, a mixture of biomass (especially heterotrophic bacteria) and substrate for this biomass. 

FIGURE 1.2. Outline of the sewer networks in separate sewered and combined sewered catchments. 

The three types of sewer networks described represent the main types. In practice, however, sanitary sewers may often appear in partially operated separate sewered catchments, i.e., they may to some extent receive runoff water. Other alternative sewer systems include, for example, the vacuum sewers that are typically small systems, operated locally. 

A gravity sewer pipe with a diameter D=0.5 m and a slope s=0.003 m m-1 is flowing half full under stationary conditions, i.e., the DO concentration is constant and equal to about 0.3 g02 m-3. The pipe is made of concrete, and the roughness is 1.0 mm. The sewer is an interceptor and serves a separate sewered catchment. The wastewater originates from domestic sources and has a temperature of T= 15°C. The characteristics of the wastewater are approximately as depicted in Figure 3.10, i.e., the potential process rates for the aerobic transformations are relatively high. Only aerobic processes in the water phase are considered in the example. 

Acid and alkaline wastes should be run as two separate sewer systems to the battery limit and to an acid treating facility or a neutralizing sump. 

The sanitary sewer constitutes a separate sewer system into which wastes of other than sanitary... 

The divided areas should be provided with sewer boxes or catch basins. The separate sewer systems are now to be connected to the properly classified sewers. Sewer box or catch basin outlet connections may be at the bottom or side depending upon the sewer invert limits (see Figure 8-3). The term invert refers to the elevation at the bottom inside surface of the sewer pipe. 

The presence of mineral oils and wastes from dry cleaning establishments makes sludge filtration difficult. Such wastes should, therefore, be kept out of the sewer system and disposed of separately. 

If the drum may occasionally receive water, caustic or similar aqueous streams, which would create problems in receiving facilities if pumped out with the hydrocarbon, then means of separate drainage should be be included. This may consist of a connection to the sewer from the bottom of the boot or in the case of sour water, a connection off the pumpout pump discharge routed to sour water facilities or other suitable disposal. 

Industrial plants also discharge domestic sewage. It is vital to keep this separate from any industrial effluent which may have to be treated, so that it can then be disposed of by conventional means (to the public sewer, septic tank, etc.). 

A sewer network and any corresponding treatment have traditionally been separately designed and operated. Two different and separate functions have been dealt with the sewer system must collect and convey the wastewater to the treatment plant, and the treatment plant must reduce pollution load into the receiving water according to the quality standards set. Consequently, sewers are often just considered input systems at the boundaries where they are connected with wastewater treatment plants and overflow structures that discharge untreated wastewater into watercourses during rainfall. This traditional approach to sewer performance needs considerable improvement. 

Biomass and substrate must be separately described to establish a concept for classification of wastewater directed toward a description of the microbial processes. For several reasons, e.g., to allow widespread application and to observe a basic mass balance, the organic matter expressed in terms of COD is a central parameter for wastewater quality. According to the concepts used in the active sludge models, the classification of wastewater in a sewer network can also be subdivided as outlined in Figure 3.1 (Henze et al., 1987, 1995a, 2000). A direct interaction between sewer and treatment plant processes is therefore within reach. 

The biodegradability within a period of time corresponding to the residence time in a sewer network (typically between a few hours and one day) must be reasonably detailed. Therefore, the fast biodegradable fractions considered as Ss and fast hydrolyzable substrate must be included as separate fractions. On the contrary, what is not biodegradable within 1-2 days is of minor interest. As a consequence, there is no need to distinguish between a rather slowly biodegradable, particulate fraction of a substrate and a fraction that is inert, whatever it is — soluble or particulate. 

A bulk water oxygen uptake rate (OUR) measurement is performed as a laboratory study. It will be separately dealt with because of its fundamental importance for sewer process studies. 

The cost of pretreating contaminated groundwater on site, for discharge to a publicly owned treatment works is often the preferred alternative (provided the facility has the capacity and local regulations allow acceptance). Pretreatment is usually required to prevent explosive vapors in the sewers and disruption of the biological treatment at the plant. The most common pretreatment includes phase separation and reduction of dissolved contaminants to an assured safe concentration. At small sites, it is not unusual to use phase separation, air stripping, and activated charcoal filtration prior to discharge to a sanitary sewer. 

---