Sewer Design

Combined sewer A sewer designed to carry both sanitary wastewaters and storm or surface-water runoff. 

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... 

ASCE (American Society of Civil Engineers) and WPCF (Water Pollution Control Federation) (1982), Gravity sanitary sewer design and construction, ASCE manuals and reports on engineering practice no. 60 or WPCF manual of practice no. FD-5, p. 275. 

The membrane separation plant is tubular ultraflltration (UF) and the pilot-plant operation was on a batch basis with a volume reduction factor approaching 40. The UF membrane had a maximum permeate flux of around 300 L/m hr at maximum 6 kg/cm inlet pressure and 3.8 m/s fluid velocity with a clean membrane. The flux typically dropped and approached 80 L/m hr at the end of a day s operation. The retentate from UF separation was returned to the feed tank whereas the permeate was routed to the sewer. Design of a full-scale plant was performed using a flux value of 40 L/m hr and volume reduction of 20x. 

Sewer design should include provision for ample future expansion of the plant and unit areas. Sewer mains, in particular, should be sized to include the estimated flow of any future expansion. 

Seppa, W.O. Fundamentals of Sewer Design. Hydrocarbon Processing Petroleum Refiner, October 1964, p. 171. 

Gravity Sanitary Sewer Design and Construction, Second Edition 62 Existing Sewer Evaluation and Rehabilitation... 

A number of flume designs have been created specifically for use in partially fiUed circular conduits such as sewers. These are available in molded fiber glass and can be lowered through a manhole if required. As with all open-channel head-area meters, flumes must be sized to prevent submergence of the restriction. 

Several special forms of electromagnetic flow meters have been developed. A d-c field version is used for Hquid metals such as sodium or mercury. Pitot and probe versions provide low cost measurements within large conduits. Another design combines a level sensor and an electromagnetic meter to provide an indication of flow within partially full conduits such as sewer lines. 

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. 

For low pressure pipelines that have ports open to the atmosphere, eg, sewers or closed effluent culverts, samplers are designed to sample through manholes. In a typical system, the Hquid is lifted through a suction line into the sampling chamber under vacuum. When filled, the vacuum shuts off, and the sample drains into a sample jar. A secondary float prevents any Hquid from reaching the vacuum pump. The suction line then drains by gravity back to the source. 

A design pressure of 545 kPa, gage is normally specified for water disengaging drums. The water outlet system is designed to seal the drum and prevent entrainment of hydrocarbon or air into the sewer. Figure 5 indicates the normal layout incorporating a single loop seal. 

In some cases, it is possible to combine the functions of blowdown and disengaging drums in one vessel. However, PR devices discharging liquid hydrocarbons lighter than pentane should not be connected into the drum if there is a possibility that such liquids could accumulate and be released to the sewer through the seal leg. Also, the drum vent should be sized to prevent pressure buildup due to vaporization. In these applications, the design criteria for both services must be met and special attention should be paid to potential hazards and problems which may be introduced, such as ... 

Note The sewer scaJ should be designed for a minimum of 175 percent of the drum s maximum operating pressure. 

Processes and equipment should be designed to reduce the chances of mis-operation by providing tight control systems, alarms and interlocks. Sample points, process equipment drains, and pumps should be sited so that any leaks flow into the plant effluent collection system, not directly to sewers. Hold-up systems, tanks and ponds, should be provided to retain spills for treatment. Flanged joints should be kept to the minimum needed for the assembly and maintenance of equipment. 

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