Versatile vacuum systems
Vacuum systems are offering water companies a viable alternative to gravity sewer systems, for some applications. WWT finds out more...
Sewerage engineers are familiar with the design and operation of gravity sewer systems. However, a number of factors including topography, soil conditions and ground water levels can make the use of such systems technically, economically and environmentally disadvantageous. In such instances, vacuum systems can offer water companies a viable alternative. This article will draw what Iseki believes are some valid environmental-impact comparisons between the two systems.
The vacuum system is built from welded PE pipework in shallow trenches, whereas the gravity system consists of spigot and socket-connected PVC or concrete pipework laid in deep trenches. The soil volume moved during the construction of a vacuum system is usually reckoned to 20-40% of that of excavated for a comparable gravity system. This means there is the potential for both environmental and societal benefits if a vacuum system is adopted.
The time required to lay a pipeline is more or less proportional to the volume of soil to be excavated. As less earth is removed during the installation of a vacuum sewer, the disruption caused during pipeline laying is reduced. The number of construction traffic movements is lessened, with encumbant benefits for local road users and the transport infrastructure.
Environmental concerns may also run to the disruption of soil strata caused by excavation. It is clear local soil conditions will be affected in greater volume and to a greater depth during the construction of gravity sewer than a vacuum one; and there is a broader environment-disturbance if well pointing is needed.
The number of pumping stations is another consideration. A gravity system can require several pumping stations dispersed along the network, sometimes at depths of up to 5m. It is usually sufficient to build a single 3m-deep vacuum collection station on a vacuum system with the discharge function. Here, again, the amount of soil excavated is reduced greatly. Gravity pipework, due to its depth, spigot and socket pipe joints and strict slope requirements is not flexible as far as the horizontal and vertical profile is concerned, so in general it must be built under the axis of features such as roads causing large scale asphalt breaking, excavation and reinstatement.
A combination of the flexible horizontal and vertical profile – which results from the hydraulic characteristics of the vacuum
sewerage system – together with the ‘easy-to-lay’ nature of the
welded polyethylene pipework and small trenches mean the pipework can be installed out of the road axis, normally in the
verge or pavement. This means both asphalt disturbance and
reinstatement work are minimal. The risk of creating weak spots
in the road surface is reduced.
Environmental advantages during operation
Principle weaknesses of the gravity system are exfiltration and infiltration. These phenomena are not problems which affect vacuum systems, Iseki believes. The spigot and socket connections of a gravity system allow ex-and infiltration, even in a new sewer. The extent of the process is regulated by national standards. The older the sewer the greater the exfiltration will be, usually as a result of changing soil structure – compaction, erosion, etc.
The bearing of a pipeline will change because of these processes, the spigot and socket connection will tighten and move,
exacerbating any existing leak and possibly affecting the surface level. Iseki believes all gravity pipework will start leaking sooner or later and its condition will worsen over time. Vacuum systems’ lack of exfiltration and infiltration protects surrounding soil from contamination and has a positive effect on the lifespan of the sewer and the built environment around it.
The elimination of groundwater infiltration causes can also have a positive influence on WwTW operation. Sewage in a vacuum-system travels much faster, sometimes reaching 20km/h, than in the gravity-network. This makes sewers ‘self-cleaning’, there is no settling, clogging or stagnation which reduces the need for jetting.
Vacuum pipe-network is sealed. The sewage enters through periodically opening, inward ventilating valves which means odour problems with vacuum systems are generally less than those associated with gravity sewers. The air outlet of the sewage collection station is usually the only part of the system which requires biofilter to treat odours. The nature of the system also means rats are unlikely.
When designing a WwTW the age and condition of the influent – including the groundwater or precipitation component – are factors to be considered. Sewage from the vacuum-pipework arrives at the WwTW quickly after entering the system and, due to the speed at which travels, as a fairly uniform effluent stream. Iseki believes this makes a big reduces the capacity requirement and environmental loading as well as the electricity-and oxygen consumption of the WwTW.