2. Active vocabulary:

3. Answer the questions:

1.What is a manhole built for? 2. What does the size of a manhole depend on? 3. What is haunching? 4. What has to be done if the frame of the manhole cover is smaller than the base?

Text 7 (2600)

1.Read and translate the text: Septic tanks

A septic tank is a brick-lined tank set into the ground, which receives the discharges from a build­ing. After treatment by natural processes, the dis­charge leaves the septic tank and filters into the soil.

If a house does not have a piped system for removing waste, then you can install a septic tank. Septic tanks can serve one plot or a small commu­nity, depending on the size. The smallest septic tank should have a capacity of at least 3000 litres or 3 m³. A 2 metre deep tank which is 2 x 2 x 1 m has an internal volume of 4 m³ or 4000 litres.

A septic tank works by using anaerobic bacteria, which do not need oxygen, to break down solid sewage into liquid and sludge. Raw sewage enters one end and emerges as a liquid effluent at the other end. A drain takes the liquid to a soakaway where it filters into the soil as you can see in Figure 3.

Inside the tank, gases cause the solids to liquefy and break up. After the solid settles as a sludge, anaerobic bacteria break it down until it forms a scum on the surface, which should be pumped out at six month intervals. The residue in the tank starts the bacterial action again in a continuous cycle.

Figure 2 Details of typical manhole construction: (a) section of a manhole excavation; (b) branch drain entering an open channel; (c) brickwork and backfill in a manhole; (d) section of a manhole showing the branch connections; (e) section of a manhole after construction.

Figure 3. Septic tank and soakaways: (a) septic tank; (b) seepage pit; (c) subsoil irrigation system; (d)subsoil drain and open joints.

The daily waste water in an average household should take about 24 hours to pass through the tank. If the water flows through too quickly, then you can put extra chambers in the tank to slow down the flow.

If a person uses 200 litres of water per day, then a household with seven people produces about 1500 litres. This volume re­quires a 3000 litre septic tank.

Septic tanks should only take sewage. Rainwater should go into soakaways. Grease from kitchen drains should be collected in a grease trap gully because it can clog up the septic tank and stop its effective operation. It is essential to clean gully traps regularly.

The location of a septic tank depends on the following principles:

• Access for pumping it out needs to be convenient.

• It should be at least 15 metres from a building;

• It should be downwind.

• The effluent should not discharge into water supplies or streams.

To build a septic tank you first need to calculate the size of a septic tank. You can see how to do this in the following example.

Example: Calculating the depth required for a 3000 litre septic tank

1. The drain enters the tank at a depth of 600 mm. The depth of liquid will be 1.5 metres.

The area of the tank should be 3 m³/1.5 m =2 m²

2. The length of the tank should be two or three times its width.

The width is 900 mm.

The length is 2200 mm.

3. The concrete base is 150 mm. Calculate the dimensions of the base in the same way as described for manholes. Note that the mini­mum wall thickness is 215 mm.

4. The depth of the excavation is 2250 mm (600 + 1500 + 150).

You follow the steps in Table 6 to build the tank.

Table 6 How to build a septic tank

Step	Action
1	Excavate the hole for the tank.
2	Cast a concrete base.
3	Build up 215 mm walls for the outside surface of the tank.
4	Build 102.5 mm internal walls.
5	Cover the top of the tank with 75 mm reinforced concrete panels, which are loosely set on the top of the walls for ventilation and to make it easy to remove scum.
6	Insert entry and exit pipes with T-junctions and extensions to push the sludge to the bottom.