Council uses three types of wastewater system to transport wastewater from connected properties to the Christchurch Wastewater Treatment Plant in Bromley, and the other, smaller treatment plants around the Banks Peninsula.
Prior to the earthquakes of 2010/11 the gravity system was the only one being used, but since then two other systems have been installed and are in use. These are, pressure, and vacuum.
These systems are new to our area but are not new technologies and both have been used around the world for many decades. They were chosen because of the advantages they have over gravity systems in the areas that they were installed in such as:
This system requires a grade or fall from one pipe to the next to allow wastewater to flow down the pipe. A property slopes downward (lateral falls) to the wastewater main in the street and the street mains fall as they work their way toward the treatment plant.
Because of the constant sloping downward to allow flow to occur the mains get deeper and deeper into the ground until such a point where they are so deep that that are very difficult to lay or repair. At this point a pump station is built with a big underground receiving tank to take the incoming flow. There are large pumps that take the wastewater out of the tank and pump it up to the next section of shallower gravity main where the wastewater journey to the treatment plant continues.
It depends on the lay of the land as to how many pump stations are required to transport the wastewater from a property to the Treatment Plant. There are some parts of Christchurch where wastewater can travel all the way to the treatment plant with no need for a pump station. In other parts the wastewater needs to be lifted or pumped several times before it finally arrives at the plant.
The closer to the treatment plant the wastewater gets the bigger the mains that are needed to transport the wastewater. This is because along the way other mains connect in and the area that the main serves gets cumulatively bigger and bigger. The larger mains are called trunk mains and in Christchurch the largest trunk main is 1.8m diameter.
In a gravity main the depth of the wastewater in the main goes up and down throughout the day, ranging from having very little at the bottom of the pipe, to being around one third to half full during. This is because the wastewater we produce from houses and businesses varies largely, from the peak periods in the morning where everyone is getting ready for work and school and using lots of water for toileting, showering washing etc to the quiet periods in the middle of the night when most of us are asleep and there are only a few businesses open.
The air inside the mains is very near to normal atmospheric pressure.
This system uses on-site property tanks with small pumps to pump wastewater out into larger, shared, street pressure mains and on to a nearby gravity system. Wastewater flows from a serviced building (house or business) into the tank via a gravity pipe (lateral). When the tank fills to a set level the pump comes on and pumps the wastewater through a pressure pipe out into a pressure main in the street which also takes wastewater from pressure tanks from the houses or businesses in the surrounding area.
The pipes from the tanks to the end of the pressure main where the wastewater is discharged into a gravity network are always under pressure. For example, if you pierced the pipe the wastewater would stream out just like water does out of a leaky water pipe. Because the pipes are under pressure they do not need to be laid as deep as gravity pipes as they do not require a grade to transport the wastewater.
The on property tanks usually only have a very small amount of wastewater in them because the level set for the pump to start is low in the tank, but they do have enough empty room (storage) to last several days without overflowing under diligent use during lengthy power cuts caused by emergencies.
There are alarm boxes attached to the tanks that alert the property owner when there is a problem with the tank so they can call the Council to respond if needed.
The materials used to make these systems, like polyethylene pipe, make them robust and resilient to future seismic events and also prevent troublesome groundwater infiltration. They are not laid deep so they are easier to install and to fix.
The pumps in the tanks can pump long distances making them an option for owners of un-serviced properties which are too far away from the nearest available wastewater system for them to lay gravity pipes on a grade to.
This system uses the vacuum created by pumps at the vacuum pump station to create a constant negative pressure in the vacuum wastewater mains which sucks wastewater from the properties connected to the station. The wastewater is then pumped using different pumps, from the receiving tank at the station to a nearby gravity system.
Wastewater travels from the buildings serviced in gravity pipes out to the street and into collector tanks which serve anywhere from 1 to 6 properties. While part of a vacuum system, this section of pipe and the collector tank are not under vacuum, they are at normal atmospheric pressure. There is a spring loaded valve in each collector tank on the pipe between the tank and the vacuum main that is normally closed but opens when wastewater rises to a set level in the tank. When it opens, the negative pressure in the main (suction) draws the wastewater from the bottom of the tank, along with some air into it. The main needs a certain amount of air for the pumps at the station to be able to create the negative pressure.
The valves in the collector tanks have controllers which control the valve opening and the amount of air allowed to enter during a valve opening. These valve controllers require periodic testing, and timing.
Like pressure wastewater, the materials used to make these systems like polyethylene pipe make them robust and resilient to future seismic events and also prevent troublesome groundwater infiltration.