The Advantages Of Pressure Sewer Systems Vs Gravity-Fed Systems

By Sally Huynh, Americas Marketing Manager, NOV | Process and Flow Technologies

As pressure increases to develop and expand our cities and communities, the need to build on areas previously deemed as unsuitable grows accordingly. This places extra demand on the sewage handling systems which we all rely on, and over time this has led to the development of some innovative technologies. One such technology is the pressure sewer system (PSS) which offers a far more reliable and controllable transfer of sewage from the household to the treatment plant than conventional alternatives. This sewage handling solution is ideal for domestic communities, and it is growing rapidly in popularity thanks to the many advantages it offers over a conventional gravity-fed system.

What Is PSS?

In essence, a PSS makes use of small, low-powered, grinder pumps in each property, which are each connected to a central discharge network.  Each of these units will have a high-density polyethylene storage tank with a typical capacity of around 900-litres (approximately 197 gallons), a progressing cavity (PC) pump, plus a cutter which breaks down the solids in the sewage to create a transferable slurry. An automatic electronic controller will also be included to regulate the flow, monitor fluid levels, and protect the pump.

The grinder ensures that the sewage is reduced to a manageable consistency before the pumps at each property transfer it away via a central ‘community’ network, usually to a pump station and on to a treatment plant.  The network serving the community is normally constructed from small diameter pipes which can be installed in narrow, shallow trenches, or via the use of directional drilling techniques. This increases the speed and safety of the installation process compared to a conventional alternative, as well as dramatically reducing overall construction costs. It also provides greater control over the network’s design and layout so that genuinely bespoke solutions can easily be created.

What Are The Benefits?

As no gravity is required to operate the system, a PSS offers controlled transfer of sewage in a far more efficient footprint than conventional sewage systems, and as such it provides unlimited possibilities. A modern PSS offers impressive performance, great reliability and the potential to be expanded as the demand for sewage handling increases.

Ease of installation also makes the PSS particularly popular for use in difficult terrain. It now becomes possible to install a network in rocky, hilly, coastal or other difficult areas, such as those with high water tables where conventional gravity-fed sewers would be too costly. A PSS offers the freedom to build and develop wherever the need exists, even in areas that were once off-limits, or those with only a low population but who must nevertheless be provided with a reliable and efficient solution.

An effective PSS will also address environmental concerns that surround many of today’s new developments. Their low impact construction characteristics minimize disruption to the local environment during installation, while their comparatively small footprint means less vegetation removal. The better systems are also completely sealed, which eliminates the opportunity for any sewage or contaminated liquid to leak into the surrounding area. Odor issues are therefore dramatically reduced by a PSS, and most systems are also perfectly compatible with effluent re-use schemes.

Operator Advantages

A PSS offers a host of other benefits too, not least the fact that it needs absolutely no input from the property owners it serves. This effectively makes a good PSS ‘invisible’ to the end user, but there are also many benefits for the network operator.

Prominent among these is the ‘intelligent operation’ capability which some systems can provide. The ability to monitor the performance of the network performance, often remotely via the web, and capture performance data allows various parameters to be managed so that the system operates at peak efficiency. Operating periods can be programmed, and staggered so that peak loads on the wider network are reduced. This feature can also help reduce or eliminate the emergency storage capacity which the network’s sewage pump station might otherwise require to cope with input surges.  This remote monitoring capability also allows for the early detection of problems in the network, so that immediate action can be taken before matters escalate, thus reducing maintenance costs and network disruption.

With some of the better PSS options available, their intelligent operation capability also includes the ability to program software upgrades and run self-diagnostic tests. Their control network and two-way telemetry features allow multiple individual units to be linked and managed from a centralized network point.

Maintaining Safety

Routine maintenance requirements are also catered for by efficient system designs. One system on the market even offers completely ‘clean’ access to the motor which sits in a dry well, as opposed to being submersed in sewage in the conventional manner. This keeps the motor and electrical systems separate from the fluid being pumped, and so makes maintenance a far quicker, safer and more pleasant process. It also ensures that unlike many alternative systems, there is no need to work in unpleasant and confined spaces. The robust nature of some of today’s grinder units also creates a zero planned maintenance requirement, which eases the task of maintenance even further.

Safety is another area where a modern PSS can offer considerable advantages. Some now include various smart safeguards which greatly reduce the possibility of people and pets coming into contact with the individual grinding and pumping unit.

What To Look For

So what features should we be looking for when we’re evaluating a possible PSS for a project? Perhaps the best starting point is to consider the two key points of a good PSS, which are its ability to grind and its ability to pump the sewage effectively. With these in mind it makes good sense to look at a PSS offered by a manufacturer who has a proven track record in both these areas.

From a practical viewpoint, a system that can deliver a high degree of flexibility is a must. One that offers a wide range of tank sizes, plus single and dual pumping options, will ensure that a bespoke network can be created to suit the size, scale and individual requirements of the development being served. This flexibility can also help improve cash flow for developers by allowing the networks to be scaled up as a development increases in size. This allows them to use a staged investment process by reducing the up-front capital costs involved.

It’s also well worth investigating whether a PSS will allow for the possibility of a hybrid PSS/gravity system to be created. If the PSS ‘plays well with others’, it can usually be made to integrate seamlessly with existing networks and installed in conjunction with other brands, without inhibiting overall performance. This provides greater control over various features of the wider network design, such as the location of pump stations.

This need for flexibility also extends to another key consideration - the need for each individual unit in a system to have a suitable emergency storage capability. The ability to store around 570 litres (150 US gallons) of sewage will be enough to protect against unforeseen emergencies such as power outages, treatment plant shutdowns and network issues, thus providing the homeowner with peace-of-mind for a 24-hour period.

The ways in which a PSS can self-monitor are another area worth considering, especially where basic parameters such as level controls are involved.  Traditional, mechanical float alternatives can be badly affected by exposure to raw sewage, whereas a system that uses solid-state level sensors with no moving parts will have no such problem, as these are designed to provide trouble-free performance for the life of the unit. Think also about any network protection features which a system offers. Events such as low voltage ’brown-outs’ and over-pressure situations can affect the performance of a system, so it pays to makes sure that the PSS you choose is designed to cope with this eventuality, preferably by means of its own microprocessor control systems.

In short, a carefully selected PSS can deliver outstanding value, reliability, flexibility and an impressively low total cost of ownership. Choose to work with a well-established provider who has a proven history with this type of sewerage solution and you won’t go wrong.

For more details visit www.inviziq.com.