Process Closed Sewers

As briefly mentioned, the occurrence of sediments in sewers is closely related to the hydraulics and physical process characteristics and the properties of the solids. This text does not include a quantitative description of the physical processes in sewers, such as sedimentation, deposition and erosion, and the corresponding physical characteristics of sewer solids. A great number of publications and textbooks address these subjects. A comprehensive description with a broad range of literature references can be found in Ashley and Verbanck (1998). Other important publications in this respect are found in Ashley (1996), Hvitved-Jacobsen et al. (1995) and Hvitved-Jacobsen (1998). Perspectives and relations to the chemical and biological sewer processes are outlined in Ashley etal. (1999). 

Waste facihties should be designed to prevent explosions in sewer systems and typically are comprised of suitable traps, vents, clean-outs, collecting chambers, etc. Flammable gas detectors are installed in sewers to warn of ha2ardous concentrations, and inert gas blanketing of closed process sumps generally is advisable. 

Block valves are installed throughout plants to return a process to a safe condition under unusual circumstances. For example, the process shown in Figure 12-2 detects a hose leak by comparing flow rates at both ends of the hose. If the hose breaks, the leak is detected and the block valves on the reactor and sewer are immediately closed. 

In addition to the redox conditions associated with the characteristics of the sewer network as outlined in Table 1.1, a number of other sewer characteristics influence the process conditions. The following examples illustrate the close relations between design and operation characteristics and process conditions ... 

Microbial transformations and generally not chemical transformations characterize the sewer environment in terms of quality transformations of the wastewater. On the other hand, the physicochemical characteristics, e.g., diffusion in the biofilm and exchange of substances across the water-air interface, play an important role and must be integrated with the microbial transformations. The hydraulics and the sewer solids transport processes have a pronounced impact on the sewer performance. These physical processes, however, are typically dealt with in hydraulics and are, therefore, only included in the text when directly and closely related to the chemical and biological processes. 

In some cases a closed drainage system can be used which drains process components directly into the oily water sewer. This has the advantage of avoiding releases of vapors in any instance, but assurance must be obtained that back pressure from one drainage location will not backfeed liquids into another drain point when two valves are open simultaneously or other drainage valves can contain any backpressure on them from other drainage sources. 

A coliform not common in papermaking systems is Escherichia coli. E. coli is closely associated with fecal contamination. If E. coli is found in the paper system, it indicates that sewer lines are entering process waters, or that faecal waste from birds or other animals contaminates the system. 

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