Installing a domestic treatment plant or cesspit

The price list item covers setting one tank, either a treatment plant or a cesspit, into a prepared excavation on a standard gravel bed. It typically includes:

• checking the base of the excavation and its dimensions against the tank,
• laying and compacting a gravel or sand and gravel bed and levelling it,
• lowering the tank into place, levelling it and checking the position of the inlet and outlet,
• connecting the inlet pipe from the house and, for a treatment plant, the outlet pipe,
• gradually filling the tank with water while backfilling around it in layers, as the manufacturer specifies,
• fitting the riser and the cover at ground level.

The job does not include the treatment plant or cesspit itself, machine excavation, removal of spoil, a concrete base slab for high groundwater, the infiltration system, the power supply for the plant's air blower or the drain pipe from the house. These are priced separately. The blower and the control unit are connected by an electrician.

Cesspit (holding tank)

A sealed tank without an outlet. It is simple and needs no electricity and no servicing of any technology. Running it is expensive, though, because everything that goes into it has to be pumped out and taken away. For a family of four, a tank of around 10 to 12 m³ typically fills within a few weeks, and every emptying is paid for separately. A cesspit therefore suits holiday homes, small households or a temporary solution until a public sewer arrives.

Domestic treatment plant

A biological plant, usually with an air blower, that cleans the wastewater and discharges it. Sludge is removed only once or a few times a year. It needs electricity, regular checks and a suitable place to send the treated water. For a house that is lived in all year round, it is usually the better choice over the long term.

Septic tank

In many countries the simplest system is still a septic tank, a settlement tank without aeration, followed by a drainage field. It needs no electricity, but it cleans the water far less thoroughly than a treatment plant and needs more land and well draining soil. Whether it is still allowed for a new house depends on local rules.

Tank material

Most tanks installed today are made of polypropylene or polyethylene. They are light and can be installed without a crane, but they are sensitive to groundwater and to incorrect backfilling. Concrete tanks are heavy and need a crane, but they resist uplift and loads better. With plastic tanks, the type matters: some are self supporting, others are designed to be surrounded with concrete. Small treatment plants sold in Europe are generally CE marked under the European standard series for small wastewater treatment systems. The manufacturer's instructions always take priority over a crew's habits.

On a new build, the plant or cesspit is usually installed together with the drain connection, once the shell of the house is finished and an excavator can still reach the plot. Later it becomes harder because of paving and garden. The best time is a dry period with low groundwater, typically summer or early autumn.

Before the tank is set, the following should be ready:

• the approval or notification required locally and a design showing the tank's position,
• the excavation at the correct depth and width, with space around the tank for backfill and with safe, supported sides,
• information about groundwater, ideally from a ground or hydrogeological survey,
• the drain pipe from the house laid at the correct fall so it arrives at the height of the tank inlet,
• a power supply for the treatment plant's blower, run in a protective duct,
• access for the tanker that will empty the tank, usually within reach of its suction hose.

The tank must keep the distances from wells, from the house and from the plot boundary that the design specifies. The distance from wells is especially important, both your own and your neighbours'. A leaking cesspit or a badly designed infiltration system can contaminate well water.

Checking the excavation

The base of the pit is cleared of loose soil, stones and standing water. The depth is checked against the height of the tank and the height of the inlet. The sides of the excavation must be stable. In deeper pits they are shored or battered back so they cannot collapse onto the tank or the workers.

Gravel bed

A layer of gravel or sand and gravel is spread over the base, typically 10 to 20 cm thick as manufacturers specify. It is compacted and screeded level with a straight edge. The tank must rest on its whole base, otherwise it will bend or crack once it is full. Where groundwater is high or the tank type requires it, a concrete slab and anchoring against uplift are used instead of a gravel bed. That is no longer a standard bed and is priced separately.

Setting the tank

A plastic tank is lowered into the pit on lifting straps, never slid down the slope. A concrete tank is set with a crane. It is turned so that the inlet faces the house and the outlet faces the infiltration system or the watercourse, and then levelled with a spirit level.

Connecting the pipes

The inlet pipe from the house is connected through the manufacturer's seal. The pipe needs a steady fall and firm bedding so the joint does not drop after backfilling. For a treatment plant, the outlet pipe and a duct for the air hose and the blower cable are connected as well.

Backfilling and filling with water

With plastic tanks this is the most important step. The tank is gradually filled with water while the backfill goes in around it in layers of about 20 to 30 cm. The water level inside should stay roughly level with the backfill or slightly above it, so the soil pressure cannot push the walls in. The backfill is sand, sand and gravel or soil free of stones, compacted by hand or with a light machine. A heavy vibrating roller next to the tank wall can damage it. Some manufacturers specify a surround of lean concrete, in which case their instructions are followed exactly.

Riser and cover

Finally the riser and the cover are set at ground level. The cover should sit slightly above the surrounding ground so that rainwater does not run in. If vehicles will drive over the tank, it needs a load spreading slab and a cover rated for that load.

Treated water from a plant has to go somewhere. If there is a stream or ditch nearby, it can sometimes be discharged into it with the consent of whoever manages the watercourse. More often it goes into the ground through an infiltration system, for example infiltration crates, a drainage field of perforated pipes or a gravel soakaway.

Infiltration only works in permeable soil. In clay the water cannot drain away, the soakaway fills up and water appears on the surface. That is why discharge into the ground is often subject to a percolation test or a hydrogeological assessment, which looks at permeability, the groundwater level and the effect on nearby wells. Infiltration for a treatment plant should not be confused with a soakaway for roof water. Rainwater is dealt with separately and must not be led into the treatment plant, because it would wash the bacteria out.

Rules for private wastewater systems differ from country to country and often from one municipality to the next, but some principles are common across Europe.

• Connection to the sewer comes first. Where a public sewer runs past the plot, connecting to it is usually compulsory if it is technically possible. A cesspit or a private plant is then generally only an interim solution. Before investing, ask the municipality whether a sewer is planned in your area.
• Emptying must be documented. A cesspit and the sludge from a treatment plant may only be emptied by a licensed or authorised contractor. Keep the receipts, because authorities may ask for them during an inspection.
• Treatment plants need maintenance. Many countries require a service contract, regular checks by a competent person or samples of the treated water. The operating manual sets out the minimum intervals.
• Approval. Check with your local authority whether the tank and the discharge of treated water need a permit or notification before you start.

A skilled self builder can install a smaller plastic cesspit, provided an excavator is available for the pit and the manufacturer's instructions are followed. For a treatment plant it is more sensible to leave installation and commissioning to the supplier, because the warranty and often the discharge approval depend on it. Steps for a cesspit:

1. Confirm any required approval and mark out the tank's position from the design.
2. Dig the pit with space around the tank and make its sides safe.
3. Pump out water from the base and remove loose soil and stones.
4. Spread sand and gravel to the thickness given in the instructions, compact it and screed it level.
5. Lower the tank on straps, turn the inlet towards the house and level it with a spirit level.
6. Connect the inlet pipe through the seal and check the fall.
7. Run a first layer of water into the tank and backfill around it to the same height with sand.
8. Compact the backfill by hand and continue in layers, always keeping the water inside roughly level with the backfill.
9. Fit the riser and cover slightly above ground level.
10. Photograph the bed, the connections and the backfill and keep the photos with your documents.

• Ignoring groundwater. An empty plastic cesspit can be lifted by groundwater in spring and tear the pipes apart.
• Backfilling without water. Soil pressure pushes in the walls of an empty plastic tank.
• Backfill with stones or rubble. Sharp stones damage the wall and the backfill cannot be compacted properly.
• An uneven or loose bed. The full tank settles unevenly, cracks or tilts the inlet.
• Wrong fall on the inlet. The house drain blocks, or the tank sits so deep that it is hard to empty.
• Infiltration in clay soil. Treated water comes up on the lawn or runs into a neighbour's basement.
• Rainwater led into the plant. After a storm the activated sludge is washed out and the plant does not clean properly for weeks.
• Cover below ground level. Rainwater runs into the tank and the cesspit fills faster.

• the tank is level and the cover sits slightly above the ground,
• the inlet and, for a treatment plant, the outlet are connected through seals and nothing is leaking,
• the walls of a plastic tank are not deformed, which you can see from inside with a torch,
• the backfill around the tank is compacted and the ground does not sink after the first rain,
• in a cesspit the level does not drop when nothing is being discharged into it, a simple tightness test,
• in a treatment plant the blower is running, aeration is visible in the tank and the control unit shows no faults,
• the emptying tanker can reach the cover,
• you have the operating manual, the warranty, the declaration of performance for the tank and photos of the installation.

Keep the photos of the bed and the backfill. If you make a warranty claim on the tank, the manufacturer will often check exactly whether it was installed according to its instructions.

In the NUMIQ price list, labour for installing a domestic treatment plant or cesspit on a standard gravel bed is €600 per unit. The price includes minor consumables but not the tank itself or the bed material, which is priced separately. Prices vary by region and city, and they are higher in large cities and areas with high demand.

The plant or tank itself, the excavation and the infiltration system usually cost considerably more than the installation labour, so it is worth asking for a complete quote that lists every item.

The price of installation and related work depends most on:

• the groundwater level and whether a concrete slab or anchoring is needed,
• the type of soil and the depth of the excavation,
• the material and weight of the tank, and so whether a crane is needed,
• the length of the inlet pipe and the type of infiltration,
• access to the plot for an excavator and a lorry.

Related work can be priced separately, for example machine excavation of a pit, trench excavation for pipes and backfilling with compaction. More items can be found under plumbing work and groundworks.