Gravity sewer systems do not work well when flows are highly variable throughout the year.
Any sewerage system should be designed for peak flow, so that it is capable of handling the maximum flow expected for the installation.
But what happens if for a large part of the year the peak flow is much lower than the design peak flow? In this case, a minimum self-cleaning velocity of the collectors is not reached for a large part of the year, causing sediments to accumulate. The result is unpleasant odours and frequent cleaning operations with vacuum trucks when the sediments completely clog the pipes.
All these problems are common in tourist towns. If, in addition, due to the proximity to the sea, there is a high water table and to reduce excavation the pipes are installed with a minimum slope, the problems are considerably aggravated.
Vacuum sewer systems are also designed for the maximum expected flow rate. However, they are less sensitive to operation at lower flow rates. Thanks to the vacuum force, high velocities are achieved even at low flow rates, so there is no risk of sedimentation. This is one of the advantages why coastal populations opt for vacuum sewers.
The opposite problem arises when the flow entering the system is much higher than the design flow of the installation. For example, if rainwater enters the wastewater system. This happens when some residents connect their rainwater to the wastewater network or when there is infiltration from the groundwater table into the wastewater network. In gravity systems, pipes can become loaded and sewage overflows can occur. This risk is again higher in flat areas with slightly sloping pipes.
In a vacuum sewer, the vacuum valves normally open only when water arrives in the pit, with as little as 40 or 50 litres. Once they have emptied the manhole, they are kept open for a couple of seconds more to allow air to enter and a mixture of air and water is conveyed at high velocity. The valves open pneumatically, with the force of the vacuum. If the flow rate is higher than the design flow rate for a prolonged period of time, too much water and not enough air can enter, reducing the system’s transport capacity.
A vacuum sewer system with monitoring of the vacuum valves reduces the impact of highly variable flows. Since the valves open pneumatically, the monitoring normally only monitors them to detect any anomalies. If necessary, however, thanks to the monitoring, some valves can be activated to open without water level and supply extra air to the system. For example, in the event of a storm warning, the air inlet could completely empty the vacuum pipe, considerably increasing its transport capacity. This would also benefit the sewage treatment plant by laminating the inflow to the treatment plant.