Formation of deposits

5

2 Deposits in combined sewer

deposits can be found and no transport takes place. The transport starts above 0.4 - 0.5 m/s when the upper layer of sediments is moved and small dunes are build. Flow veloc-ities of 1 - 2 m/s show a strong transport of solids. All sediment layers are moved along the bottom in shape of dunes. More solids are jumping (saltation), rolling and sliding along the sewer bottom. The maximum transport rate is reached for flow velocities of 3 - 4 m/s when all solids are in suspension and no deposition is given. The concentration of solids in the flow is homogenous to a large extend. Despite the strong connection of moving and fixed deposits both categories will be described individually in the following sections.

Local deposits can be divided into long stretched bodies of solified sediments and sedi-ments in front of obstacles or hydraulic unfavourable locations. Investigations of Risten-part (1995) showed sediment levels of 5 - 20 cm in a sewer of 1500 mm diameter, which means 3.3 % to 13.3 % of the sewer diameter were affected by deposits. (Figure 2.2)

Figure 2.2: Longitudinal view of sewer deposits, [Ristenpart, 1995]

Two categories of local deposits can be classified. Blumberg & Bauer (1984) investi-gated an egg shaped sewer of 800/1200 mm diameter. They identified deposits with a high organic fraction and a spongy consistency. The content of the dry substance was measured with 10 - 40 %, the ignition loss was 25 - 60 %. Additionally deposits with a mainly mineral fraction (sand) and high compression strength were found. The content of the dry substance was higher with 50 - 80 % and the ignition loss was smaller with 5 - 20 %.

Perrusquia et al. (1995) found sediments consisting of coarse material at the sewer bottom as a first layer and a layer of fine material above (top organic layer). The char-acteristic and the movement of the found sediments seemed to be accidental and varied strongly.

2.1.2 Conditions for sedimentation

The reasons for the formation of sedimentations of sewer solids are the design of the sewer system, external factors of influence and the deposits itself. The parameters, which lead to sedimentation of deposits, will be shown in the following.

The hydraulic boundary conditions in a sewer system regulate the flow velocities and the bottom shear stresses, which are responsible for sedimentation and erosive processes.

The slope of a sewer channel, the shape and diameter of the pipe, the roughness and

Formation of deposits 7 the line management of the channels influence the sedimentation as well as changes in profile, inlets, throttles and combined sewer overflows. The density, the size and the shape are also responsible for the sedimentation or the start of the movement due to changed flow conditions. [Schmitt, 1992]

During dry-weather periods the rate of sedimentation increases due the reduced flow ve-locity. Depending on the characteristics of the sewer this can lead to a reduced hydraulic radius and increased bottom roughness, which again slows down the flow velocity and favours the sedimentation. Seasonal changes in water consumption or the reduced inflow of industrial water at the weekends also influence the sedimentation processes. This ap-plies for the connected catchment, which changes his entry of sediments into the sewer system during the year. In the wintertime more mineral sediments will enter the sewer channels while in the summer and spring the fraction of organic material will grow.

2.1.3 Areas of sedimentation

Based on the conditions of sedimentation as described in the prior section the typical areas of sewer solid sedimentation can be defined. Locations for the formations of de-posits can be places with adverse hydraulic flow conditions like junctions, openings and strong bends in sewer channels where the flow velocity is reduced. Small sewers in the beginning of a system can be affected by small runoffs while the large sewers at the end of the system often have a low bed slope, which also favours the start of deposition.

Siphons are sewer buildings, which usually show different slopes in their curvature and are strongly affected by sedimentation. [Westrich, 1984]

A good example for the deposition of sewer solids in a channel narrow gives the inves-tigation of Brombach et al. (1993). The collector had an initial diameter of 1600 mm with a mean bottom slope ofI=2.6 . After 110 m length the sewer was reduced to a diameter of 0.8 m width and 1.8 m height. A backwater area is formed and the connected low flow velocities are responsible for the sedimentation. After a general cleaning during a period of six-month deposits with a thickness of 20 cm were built up. The content was predominately organic material with some mineral parts like sand or stones.

Settlements of sewer pipes or sewer buildings can also be responsible for the sedimen-tation of sewer solids. Beside the problem of infiltration water small steps are created along the sewer pipe, which create a hydraulic loss. The flow velocity and the shear stresses are reduced at these locations, while sedimentation can start then very easily under these conditions. Large steps may create a backwater with dead areas, which also helps the sewer solids to settle down. Brombach et al. (1992) found in his investigations sedimentations created by steps in sewer pipes.

A further example is the depositions investigated by Westrich (1984) in a reservoir sewer with an upstream combined sewer overflow (CSO). Sedimentation occurred especially at the end of a storm event when the runoff decreased and the critical flow velocity u_crit was approached. The areas 1 and 2 show an increase of sedimentation and the thickness of the deposit layer. At the CSO the flow velocity was increased and therefore the sedi-mentation was reduced. Further downstream at the end of the reservoirs the sewage was throttled to a constant runoff and the flow velocity was reduced again. Therefore the areas 3 and 4 acted like a sand trap. Figure 2.3 shows the flow condition in the reservoir

sewer.

Figure 2.3: Longitudinal view of sewer deposits in reservoir sewer [Westrich, 1994]

2.1.4 Mobile deposits

Due to their physical properties sewer solids can be assigned to certain types of transport.

• Suspended particles move with the flow.

• Floating particles move ahead of the flow.

• Coarse suspended particles sail in the flow.

• Fine suspended particles settle down slow.

• Settleable particles can be transported by turbulent flows.

• Particles slide or bounce slowly along the sewer bottom.

• Sand particles wallow forward in dunes.

• Gravel and stones belong to fixed sediments.

Figure 2.4 displays the different ways of particle movements. The formation of con-glomerates or flakes or adsorption of dissolved particles can lead to different forms of movement.

Content of sewer deposits 9

Figure 2.4: Particle movement, [Glazik, 1989]

The transport of sewer solids is composed of bed load and suspended transport. The bed load is the main source of sediments deposited on the sewer bottom and consists of highly concentrated organic material with fibres like toilet paper. [Ristenpart, 1995]

The following figure 2.5 shows the bed load transport with the distribution of velocity and concentration of suspended solids during dry-weather runoff.

Figure 2.5: Bed load transport during dry-weather runoff, [Ashley et al., 1992]

Investigations on bed load transport divided the moved solids in two groups. First the small forms of deposition, which are ripples on the sewer, bottom and second the dunes or bars of sediments. Sander (1989) gives a good overview on the different shapes of mobile depositions.