Sewer solids

Because of the basic requirements of collection and transport, sewer networks are normally dealt with from a physical point of view, i.e., the hydraulics and sewer solids transport processes have been focal points. From this point of view, new design and operational principles have been developed, to a great extent supported by numerical procedures and the ever-increasing capacity of computers. Under wet-weather conditions, the hydraulics and solids transport phenomena in a sewer play a major role, and the chemical and microbiological processes are typically of minor importance. Not surprisingly, interests devoted to urban drainage have focused on the physical behavior of the sewer. 

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. 

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). 

Most sanitary and combined sewer networks consist of pipes designed to flow as open channels, i.e., with a free water surface. The wastewater flows downstream in such pipes by the force of gravity with a velocity of flow that depends principally on the pipe slope and frictional resistance. Typically, the design velocity is between 0.6 and 3 m s-1 to avoid blockage of the pipe by sewer solids accumulated at low flow conditions and to prevent damage of the sewer at a high flow. 

Wastewater in sewers includes different and varying species of heterotrophic microorganisms. A simple relationship between biomass growth and substrate utilization is needed. Several studies performed with different types of wastewater and sewer solids have shown that a simple description is possible and acceptable (Bjerre et al., 1995 Vollertsen and Hvitved-Jacobsen, 1999). 

Depending on sewer design and hydraulic conditions, sewer solids may temporarily or more permanently accumulate as sediments in gravity sewer networks (cf. Section 3.2.8). In sanitary sewers, this may depend on the daily... 

The concrete sewer network or pumping station that is considered is located downstream of systems with risk for a high sulfide formation. Such systems may primarily include pressure mains and also gravity sewers with permanent deposits of sewer solids. 

Vollertsen, J., M. do C. Almeida, and T. Hvitved-Jacobsen (1999), Effects of temperature and dissolved oxygen on hydrolysis of sewer solids, Water Res., 33(14), 3119-3126. 

Wet-weather processes have, in general, been excluded in the text, because they are based on a different concept and perform differently. Microbial and physicochemical processes are contrary to the physical processes dominating in sewers during dry-weather transport of the wastewater. When dealing with combined sewer networks in terms of pollutant loads during overflow events, dry-weather solids deposition and erosion and solids transport during high-flow events are, in addition to the rainfall/runoff hydraulic and sewer solids characteristics, the central physical in-sewer processes. Quite different process approaches are, therefore, required to describe dry-weather and wet-weather sewer performance. 

The environmental issues faced by the pharmachemical manufacturer are no different from that with other fine organic chemical producers. The concerns cover all three possible routes of environmental contamination liquid waste to sewers, solid waste generated at the site, and air pollution. 

Wastewater treatment plants are also a significant source of refinery air emissions and solid wastes. Air releases arise from fugitive emissions from the numerous tanks, ponds, and sewer system drains. Solid wastes are generated in the form of sludges from a number of the treatment units. 

Sewage The used water and water-carried solids from homes that flow in sewers to a wastewatef treatment plant. The preferred term is wastewater. 

For most smaller operators, the waste water discharge from pretreatment equipment, blowdown receivers, and FSHR equipment typically discharges into a city sewer. Most larger factories, process plants, and power stations, on the other hand, incorporate some form of waste water treatment facility to balance the pH level, remove oils and pre-cipitable solids, or otherwise reduce the contamination load before the discharge of water from the site. 

CONFINED SPACE A spacc which is substantially, although not always entirely, enclosed and where there is a reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. The risks may include flammable substances oxygen deficiency or enrichment toxic gases, fume or vapour ingress or presence of liquids free-flowing solids presence of excessive heat. For the purpose of the Confined Spaces Regulations 1997 a confined space means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk. 

Another pathway by which targeted pollutants enter the wastewater stream is through the disposal of spent batch process solutions into the sewer system. Spent solutions consist of aqueous wastes and may contain accumulated solids as well. Spent solutions are typically bled at a controlled rate into the wastewater stream. Other sources of pollutants in wastewater streams include cleanup of spills and washdown of fugitive aerosols from spray operations. 

Clarified water is removed by effluent extraction pipes that are attached to the moving center section. The clarified water that normally contains <30 mg/L of suspended solids can be recycled in the process and/or sewered. 

While primary treatment is always used in municipal plants, because no one knows what might end up in a city sewer, it may be eliminated from industrial plants 33 when the suspended solids are less than 125 mg/1 and oils and greases are less than 50 mg/1. 

As previously mentioned, the text will primarily focus on the microbiological processes in the sewer and, to some extent, also the chemical processes, especially the physicochemical processes. The corresponding environmental engineering relevance will be included as being the ultimate goal. Hydraulics and solid-transport phenomena, installation details, materials and the traditional... 

The development that has taken place is the result of 100-150 years of enormous investments. All over the world, this has left us with a sewer and treatment plant infrastructure that will be in use for an unknown future. We will still see developments in terms of technical improvements and sustainable solutions. However, we will not, as a general trend, see the wastewater collection and treatment concept replaced by, e.g., centralized collection of solid human excreta or on-site solutions. This could have been a realistic option for further development 150 years ago — not now ... 

The objective of this chapter is to highlight fundamental chemical and physicochemical aspects of general importance for sewer systems and in-sewer processes. The contents are selected with this in mind, and the focus is on chemical and physicochemical concepts adapted to this purpose. The chapter is written to serve as a solid background for understanding process-related aspects considering the sewer as a reactor. 

The microbial processes proceed in different subsystems of the sewer the suspended wastewater phase, the biofilms, the sediments and the solid surfaces in contact with the air phase. 

The classification of wastewater in terms of size distribution is normally done from a practical point of view. Typically, a distinction is made between soluble, colloidal and suspended components (Figure 3.6). While this definition for determining what solids are is rational as far as physical transport processes in sewers are concerned, when dealing with the microbial processes for sewer conditions, an extension of the solids definition is required. Particles larger than about 10-4 pm cannot be transported through the cell wall and are, therefore, from a microbial point of view, considered particles. 

The occurrence of sewer sediments is primarily determined by the physical characteristics of wastewater solids and the hydraulic conditions. Basically, sewers should be designed and operated in a way that does not result in permanent deposits. This ideal performance of a sewer is not generally observed, and sediments may be more or less temporarily accumulated in sewers. In combined sewer networks, sediments may settle under dry-weather conditions when the wastewater velocity and shear stress at the bottom are low and be... 

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