Gravity sewers

Gravity Sewer Pipe This pipe is made in five classes for varying depths or bury, trencn dimension, soil, and vehicular loading (Table 10-37). 

TABLE 10-37 Asbestos-Cement Gravity Sewer Pipe ... 

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. 

Aerobic + Oxygen Partly filled gravity sewer Aerated pressure sewer... 

Sulfate (+co2) Gravity sewer with low slope and deposits... 

Wastewater is transported under aerobic conditions in a half-full intercepting gravity sewer pipe for 4 hours. It is assumed that transformation of the organic matter only proceeds in the wastewater phase and follows a 1-order removal kinetics. 

FIGURE 3.2. Outline illustrating the subsystems and occurrence of microbial processes in a gravity sewer under anaerobic conditions. 

Based on the conversion factors given in Table 3.3, the composition of wastewater samples taken at the inlets of four wastewater treatment plants in Denmark is shown in Figure 3.7. The wastewater in the corresponding sewer catchments mainly originates from domestic sources, and the network mainly consists of gravity sewer sections. The results show that the three components — carbohydrates, proteins and lipids — make up a significant part of the... 

The transformation of carbohydrates, proteins and lipids has been investigated during transport in an intercepting gravity sewer under aerobic conditions. It was seen that dissolved carbohydrates and, to some extent, proteins were removed, whereas the concentration of lipids was almost unchanged (Figure 3.9). 

FIGURE 3.9. Concentrations of dissolved carbohydrate and protein in wastewater during transport in a gravity sewer under aerobic conditions (Raunkjaer et al., 1995). 

Significant biomass production can take place in a gravity sewer biofilm. The biomass generated in the biofilm detaches and is, together with the biomass produced in the water phase, transported to the treatment plant or via overflow structures into receiving waters. A simple method to assess the amount of... 

FIGURE 3.12. Typical composition of a gravity sewer biofilm (Jahn and Nielsen, 1998). 

Hvitved-Jacobsen, T., J. Vollertsen, and P.H. Nielsen (1998), Aprocess and model concept for microbial wastewater transformations in gravity sewers, Water Sci. Tech., 37(1), 233-241. 

Nielsen, P.H., K. Raunkjaer, N.H. Norsker, N.Aa. Jensen, and T. Hvitved-Jacobsen (1992), Transformation of wastewater in sewer systems —Areview, Water Sci. Tech., 25(6), 17-31. Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7),... 

The value of the overall oxygen transfer coefficient, KLa = KLC>2, is central for the determination of the rate of oxygen transfer, F. Table 4.7 summarizes a number of empirical expressions proposed for the determination of K,a in gravity sewers (Jensen, 1994). 

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). 

TABLE 4.7. Empirical Expressions for the Determination of the Overall Oxygen Transfer Coefficient, KLa (20) = KLOa(20), in Gravity Sewers. 

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. 

TABLE 4.8 Empirical Expressions for Determination of the DO Deficit Ratio, So, for Reaeration at a Sewer Fall in a Gravity Sewer. 

Hwang, Y., T. Matsuo, K. Hanaki, and N. Suzuki (1995), Identification and quantification of sulfur and nitrogen containing odorous compounds in wastewater, Water Res., 29(2), 711-718. Jensen, N.Aa. (1994), Air-water oxygen transfer in gravity sewers, Ph.D. dissertation, Environmental Engineering Laboratory, Aalborg University, Denmark. 

Almeida (1999) made transformation studies of wastewater components in a gravity sewer. The sewer has a length of 7.2 km and a typical retention time of 1.5 hours. An average slope equal to 0.007 and several drops resulted in a sewer dominated by aerobic processes. In addition to the organic components (CODtot, CODsol and BOD), other relevant parameters (ammonia, nitrate, TSS... 

TABLE 5.1. Estimated Average Removal Percentages of Selected Wastewater Components in a 7.2-km Gravity Sewer with an Average Retention Time of about 1.5 h. The Sewer Processes are Predominantly Aerobic (Almeida, 1999). 

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. 

TABLE 5.3. Matrix Formulation of Aerobic Microbial Transformations of Wastewater Organic Matter in a Gravity Sewer (cf. Figure 5.5). The Formulation that is Shown Includes Two Fractions of Hydrolyzable Substrate. 

FIGURE 5.10. Variability over day and night of the DO concentration in a gravity sewer. The variability during the three days and nights corresponds to different temperatures of the... 

FIGURE 5.11. Duration of anaerobic conditions in an intercepting gravity sewer over a period of time with a wastewater temperature that increases from 9-14°C. 

Gudjonsson, G., J. Vollertsen, andT. Hvitved-Jacobsen (2001), Dissolved oxygen in gravity sewers — measurement and simulation, Proceedings from the 2nd International Conference on Interactions between Sewers, Treatment Plants and Receiving Waters in Urban Areas (INTERURBAII), Lisbon, Portugal, February 19-22, 2001, pp. 35-43. 

Jensen, N.Aa. and T. Hvitved-Jacobsen (1991), Method for measurement of reaeration in gravity sewers using radiotracers, Research Journal WPCF, 63(5), 758—767. 

Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7), 159-167. 

Raunkjaer, K., T. Hvitved-Jacobsen, andP. H. Nielsen (1995), Transformation of organic matter in a gravity sewer, Water Env. Res., 67(2), 181—188. 

The oxidation of sulfide to elementary sulfur (S) or sulfate (SO4-) may take place when aerobic conditions exist. If sulfide is produced in the deep part of a biofilm in a gravity sewer, it may be oxidized in an aerobic upper layer of the biofilm or in the water phase (Figure 6.2). The details of the oxidation are not well known and may be due to chemical and biological processes. The final step of this process is sulfate, although sulfur in an oxidation step of 0 may be temporarily generated. Oxidation of sulfide that is released into the sewer atmosphere will be dealt with in Section 6.2.6. 

Referring to Figure 6.2, it is important and interesting to note that a DO consumption that takes place in a gravity sewer biofilm may proceed with relations to aerobic and anaerobic processes. The anaerobic microbial processes produce... 

FIGURE 6.2. Aerobic and anaerobic process interactions in a gravity sewer biofilm. 

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. 

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