Gravity-flow sewers

In components installed underground, particularly gravity-flow sewers, the temperature of both the waste water and the air space has a marked influence on the biogenic corrosion reactions occurring on the sewer walls above the level of the water. 

Ductile sewer pipes are standardised in DIN EN 598 for gravity-flow sewers and forced-flow pipelines pressurised up to 6 bar (600 kPa) in the nominal diameters DN 100 to DN 2000 [20]. They are used for combined, sanitary or storm water pipe-... 

When designing a pressurized pipe system, the pipe selected must hold the internal pressure safely and continuously. In a non-pressurized system such as a gravity flow sewer, pipe selection depends on other factors. Vacuum piping systems must use a pipe that resists collapse. The design engineer will use different design criteria and calculations for each type of installation. 

An adequate drainage system should be provided for all locations where a large amount of hydrocarbon liquids has the possibility of release and may accumulate within the terms of the risk analysis frequency levels. Normal practice is to ensure adequate drainage capability exists at all pumps, tanks, vessels, columns, etc., supplemented by area surface runoff or general area catch basins. Sewer systems are normally gravity flow for either sanitary requirements or oily surface water disposal. Where insufficient elevation is available for the main header, lift stations are installed with a forced pressure outlet header to a disposal or treatment system. 

Sewers, in general, are designed for gravity flow. In a tightly sealed system, a rise in water level will reduce the vapor space and cause an increase in pressure. Such a tendency will, in turn, reduce the sewer s design capacity. Under these circumstances, vents are necessary to release vapors and to prevent vapor lock. Vents are functionally designed to maintain atmospheric pressure in the sewer and to release vapors to safe locations. 

Large-diameter profile wall sewer and drain pipes are used in low pressure and gravity flow applications, while EN 1636-5 includes specifications for pipes and fittings of polyethylene piping system for nonpressure drainage. 

A determination should be made of the estimated life of the installed thermoplastic pipe system. When designing a city sewer system a life expectancy of 50 years is normal. A temporary, gravity-flow, mine slurry line may be in service only five years before operations are moved. A specialized chemical process plant may be obsolete or renovated in 15 to 20 years due to technology changes. The design parameters will vary depending on the intended use and desired life expectancy. 

Gravity Flow systems are common in industrial and municipal waste and sewer lines as well as water and slurry pipelines. Gravity flow systems may operate under full flow or partially full conditions. Because of the superior wall smoothness and excellent flow characteristics of thermoplastic pipe, they are an excellent choice for gravity flow piping systems. 

The flow in sanitary sewers may be controlled by gravity (gravity sewers) or pressure (pressure sewers). In a partially filled gravity sewer, transfer of oxygen across the air-water interface (reaeration) is possible, and aerobic heterotrophic processes may proceed. On the contrary, pressurized systems are full flowing and do not allow for reaeration. In these sewer types, anaerobic processes will, therefore, generally dominate. 

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. 

The expressions in Table 4.7 show that sewer systems and flow characteristics determine the magnitude of KLa. Figure 4.5 illustrates how K,a varies with the flow in a gravity sewer with a diameter D = 0.7 m and a slope s = 0.003 at a temperature of 15°C. The figure also depicts the corresponding water depth-to-diameter ratio (y/D) and a full-flowing pipe at about 530 m3 h 1 (1471 s-1). 

FIGURE 4.5. KLa and water depth-to-diameter ratio (y/D) versus the flow in a gravity sewer pipe with a diameter 0 = 0.7 m and a slope s = 0.003 at a temperature 15°C. 

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. 

Anaerobic conditions prevail in full-flowing gravity sewers and pressure mains. In cases where aerobic wastewater flows into such sewers, the DO concentration is typically fast depleted, often after 10-30 minutes, depending on the level of the DO concentration and the aerobic respiration rate of the wastewater. Although sulfide problems in sewer networks are particularly widespread in countries with high temperatures, it may also occur in pressure mains during winter under temperate climate conditions, i.e., at temperatures around 5-12°C (Hvitved-Jacobsen et al 1995 Nielsen et al 1998). Under such low temperature conditions, the sulfide production rate is low, and the anaerobic residence time should typically exceed 0.5-2 hours before sulfide production is significant. 

The first term in Equation (6.8) is basically equal to Equation (3) in Table 6.1, and thereby corresponds to the formation rate of sulfide. However, the areal sulfide formation rate is normally found to be lower in a gravity sewer than in a pressure main, probably because of the effects of the daily changing water level and flow conditions. The value of M is therefore typically lower than shown in Equation (3). Pomeroy and Parkhurst propose M =0.3210-3 m h-1. 

Systems that are exposed to excessive turbulence of anaerobic wastewater and a potential increased release of hydrogen sulfide. Systems with a risk for increased turbulence are inlet structures, drops, cascades, sharp bends and inverted siphons. As an example, changes in the flow regime from a pressure pipe into a gravity sewer may give rise to the release of hydrogen sulfide. Corrosion of the sewer pipe wall is often pronounced near the daily water... 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

For aerobic gravity sewers, procedure 4 is the ultimate calibration of the sewer process model. This is based on procedures 1 to 3 using information from upstream and downstream wastewater samples and by including local sewer systems and flow characteristics, temperature and DO concentration values of the wastewater in the sewer. Example 7.2 outlines the results of calibration and validation performed on a 5 km intercepting sewer line. 

The example illustrates how the flow conditions of a sewer pipe affect the reaeration and the resulting DO concentration in the wastewater. As an example, a gravity sewer with a pipe diameter of 500 mm and a slope s=0.003 m nr1 is selected. The sewer is without deposits but with a biofilm on the wetted perimeter. The DO consumption rate of the bulk water phase, rw, is at 10°C assumed to have a maximum value equal to 5 g02 nr3 h 1, however, is limited by the magnitude of the reaeration. The DO consumption rate of the biofilm, rf, is considered a 1-order process in the DO concentration by following Equation (5.12). 

In Figure 8.3, the oxygen transfer coefficient, KLa, the flow velocity, u, the bulk water DO concentration and the DO consumption rate of the biofilm, ry, are all plotted versus the flow, Q, under steady state conditions in a gravity sewer pipe under the conditions given. 

The hydraulic performance of sewer pipes can be described at different levels. In the case of nonstationary, nonuniform flow, the Saint Venant Equations should be applied. However, under dry-weather conditions, the Manning Equation is an adequate description of the wastewater flow in a gravity sewer pipe when considering the prediction of wastewater quality changes under transport. There are no grounds for using advanced hydraulic models because of the uncertainties in the prediction of the microbial transformations of the wastewater. 

FIGURE 8.7. A 50 km intercepting gravity sewer from Dortmund to Dinslaken, Germany. The slope of the sewer is generally >0.13%. The total daily average dry-weather flow into the interceptor is 4.8 m3 s-1. 

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