Chapter Four: Off-Site Sanitation Systems

1 Chapter Four: Off-site Sanitation Systems

Off-site sanitation refers to a sanitation system in which wastewater and excreta are collected and conveyed away from the plot where they are generated. An off-site sanitation system relies on a sewer technology (simplified sewer, solid free sewer or conventional sewer) for conveyance of excreta.

1.1 Decentralized Wastewater Treatment Systems (DEWATS)

Decentralized wastewater management systems (DEWATS) include all parts of a onsite-sanitation system. In comparison to centralized systems, these systems are located at or near the point of wastewater generation. DEWATS can be characterized and differentiated from centralized systems along the following lines.

• Volume: Decentralized systems treat relatively small volumes of water (typically 1-1,000 m³/day),
• Sewer type: Centralized systems typically use conventional gravity sewers, while decentralized systems typically use small-diameter gravity sewers, often employing intermediate settlers for solid-free sewers.

1.2 Components of DEWATS

The components of DEWATS are presented in the schematic layout shown in Figure 4.1

Figure 4.27: Treatment Flow Sheet for Components of DEWATS (Source: MoW, 2018)

1.2.1 Containment

Containment technologies collect and store wastewater at the user interface on-site. Containment technologies are usually applicable for low-cost, non-sewered sanitation (FS) systems as intermediate storage, but can also serve as pre-treatment modules for small-scale wastewater treatment systems. The main containment technology applicable for wastewater treatment technologies is a septic tank (see Figure 4 .28).

Figure 4.28: The Cross Section of a Septic Tank (Source: MoW, 2018)

In the vast majority of situations, containment systems are already installed on-site but are often improperly designed, constructed and maintained, which poses severe environmental hazards. Apart from septic tanks providing some degree of pre-treatment, the effluent usually contains high concentrations of pollutants, which can carry severe public health and environmental burdens, especially in densely populated urban areas and in the vicinity of drinking water sources. Hence, proper sealing of containment options is crucial for environmental sanitation. Containment systems can also be implemented to buffer peak flows.

1.2.2 Design of Septic Tank

The capacity of septic tank depends on number of users and interval of sludge removal. Normally sludge should be removed every 2 years. The liquid capacity of the tank is taken as 130 litres to 70 litres per head. For small number of users 130 litres per head is sufficient.

A septic tank is usually provided with brick walls in which cement mortar [not less than 20 cm (9 inch)] thick and the foundation floor is of cement concrete 1:2:4. Both inside and outside faces of the wall and top of the floor are plastered with minimum thickness of 12mm thick cement mortar 1:3 mix.

All inside corners of the septic tank are rounded. Water proofing agent (such as Impermo, Cem-seal or Accoproof, etc.) is added to the mortar at the rate of 2% of the cement weight. Water proofing agent is to be added in similar proportion in to the concrete also for making the floor of the tank.

For proper convenience in collection and removal of the sludge, the floor of the septic tank is given a slope of 1:10 to 1:20 towards the inlet side. Which means that the floor of the outlet side will be on the higher elevation than the floor at the inlet side.

1.2.2.1 Dimensioning a Septic Tank

(A) Length, Width and Depth of Septic Tank
Width = 750 mm (minimum) Length = 2 to 4 times width Depth = 1,000 to 1,300 mm. (min below water level) + 300 to 450 mm free board Maximum depth = 1,800 mm + 450 mm free board Capacity = 1 cubic metre (minimum)

(B) Detention period
Detention period of 24hrs (is mostly) considered in septic tank design. The rate of flow of effluent must be equal to the rate of flow of the influent.

(C) Inlet and outlet pipes
An elbow or T pipe of 100mm diameter is submerged to a depth of 250-600 mm below the liquid level. For the outlet pipe an elbow or T type of 100mm diameter pipe is submerged to a depth of 200-500 mm below the liquid level. Pipes may be of stone ware or asbestos or PVC.

(D) Baffle Walls of the Septic Tank
For small tanks, RCC hanging type scum baffle walls are provided in septic tanks. Baffle walls are provided near the inlet. It is optional near the outlet. The inlet baffle wall is placed at a distance of L/5 from the wall, where L is the length of the wall. The baffle wall is generally extended 150 mm above to scum level and 400-700 mm below it.

Scum being light, generally floats at the water level in the tank. Thickness of the wall varies from 50 mm to 100 mm. For large tanks the lower portion has holes for flow of sludge.

(E) Roofing Slab of the Septic Tank
The top of the septic tank is covered with a RCC slab of thickness of 75-100 mm depending upon the size of the tank. Circular manholes of 500mm clear diameter are provided for inspection and desludging. In case of rectangular opening clear size is kept as 600 x 450 mm.

(F) Ventilation Pipe
For outlet of foul gases and ventilation purpose cast iron or asbestos pipe of 50-100 mm diameter is provided which should extend 2m (minimum) above ground level. Top of the ventilation pipe is provided with a mosquito proof wire mesh or cowl.

1.2.3 Conveyance

Technologies presented in this section are sewer-based technologies, using water from waterborne toilets as a conveying medium.

1.2.3.1 Simplified Sewer

A simplified sewer describes a sewerage network that is constructed using smaller diameter pipes laid at a shallower depth and at a flatter gradient than conventional sewers (Figure 4.3).

Figure 4.29: Sketch of a Simplified Sewer (Source: MoW, 2018)

This sewer system generally does not apply pumping. For this reason, a simplified sewer allows for a more flexible design at lower cost. Simplified sewers can be installed in almost all types of settlements and are especially appropriate for densely populated urban areas where space for on-site technologies is limited. The sewers should be considered as an option where there is significant population density (about 150 inhabitants per hectare) and a reliable water supply system (at least 60 L/capita/day).

1.2.3.1.1 Design considerations for simplified sewers

In contrast to conventional sewers that are designed to ensure a minimum self-cleansing velocity, the design of simplified sewers is based on a minimum tractive tension of 1 N/m2 (1 Pa) at peak flow. The minimum peak flow should be 1.5 L/s and a minimum sewer diameter of 100 mm is required. A gradient of 0.5% is usually sufficient. For example, a 100 mm sewer laid at a gradient of 1m in 200 m will serve around 2,800 users with a wastewater flow of 60 L/person/day. PVC pipes are recommended for use. The depth at which they should be laid depends mainly on the amount of traffic. Below sidewalks, soil covers of 40 to 65 cm are typical. The simplified design can also be applied to sewer mains. They can also be laid at a shallow depth, provided that they are placed away from traffic.

Expensive manholes are normally not needed. At each junction or change in direction, simple inspection chambers (or cleanouts) are provided as can be seen on Figure 4 .29. Inspection boxes are also used at each house connection. Where kitchen grey water contains an appreciable amount of oil and grease. The installation of grease traps is recommended to prevent clogging. Grey water should be discharged into the sewer to ensure adequate hydraulic loading, but storm water connections should be discouraged. In practice it is difficult to exclude all storm water flows, especially where there is no alternative for storm drainage. The design of sewers (and treatment plants) should, therefore, take into account the extra flow that may result from storm water inflows.

1.2.3.1.2 Design Procedures for Simplified Sewers

Step 1. Estimation of the wastewater flow
Daily peak flows
The value of the wastewater flow used for sewer design is the daily peak flow. This can be estimated as follows:

where: q = daily peak flow, l/s k1 = peak factor (=daily peak flow divided by average daily flow) k2 = return factor (=wastewater flow divided by water consumption) P = population served by length of sewer under consideration w = average water consumption, litres per person per day and 86,400 is the number of seconds in a day.

A suitable design value for k1 for simplified sewerage is 1.8 and k2 may be taken as 0.85. Thus, equation 4.1 becomes:

Variations in the value of k2 have a much lower impact on design, except in middle and high-income areas where a large proportion of water consumption is also for lawn-watering and car-washing. In peri-urban areas in Brazil a k2 value of 0.85 has been used successfully, although other counties use a value of 0.65, even in low income areas and without any reported operational problems (Luduvice, 2000). However higher values may be more appropriate elsewhere–for example, in areas where the water supply is based on a system of public standpipes, values up to 0.95 may be used.

Step 2. Sizing of a simplified sewer
The flow in simplified sewers is always assumed to be an open channel flow–that is to say, there is always some free space above the flow of wastewater in the sewer. The hydraulic design of simplified sewers requires knowledge of the area of flow and the hydraulic radius. Both these parameters vary with the depth of flow.

From Figure 4 .30 shows the trigonometric relationships which can be derived for the following parameters:

(i) The area of flow(a), expressed in m2;
(ii) The wetted perimeter(p), m;
(iii) The hydraulic radius(r), m; and
(iv) The breadth of flow (b), m.

The hydraulic radius (sometimes called the hydraulic mean depth) is the area of flow divided by the wetted perimeter.

Figure 4.30: Definition of Parameters for Open Channel Flow in a Circular Sewer (Source: Mara, 1996)

If the angle of flow is measured in degrees, it must be converted to radians by multiplying by (2π/360), since 360o equals 2π radians.

The ratio d/D is termed the proportional depth of flow (which is dimensionless). In simplified sewerage systems the usual limits for d/D are as follows:

0.2 <d/D < 0.8

The lower limit ensures that there is sufficient velocity of flow to prevent solids deposition in the initial part of the design period, and the upper limit provides for sufficient ventilation at the end of the design period. The equations are as follows:

Angle of flow, θ = 2 cos^-1 [1 – 2 (d/D)] ......................................(4.3)

Area of flow, a = D² [(θ – sin θ) / 8] .......................................(4.4)

Wetted perimeter, p = θ D/2 ...........................................................(4.5)

Hydraulic radius (= a/p), r = (D/4) [1 – ((sin θ) /θ)]....................................(4.6)

Breadth of flow, b = D sin (θ/2), .......................................................(4.7)

When d = D (that is, when the sewer is flowing just flow), then a = A = π D2/4; p = P = πD and r = R = D/4. The following equations for ‘’a’’ and ‘’r’’ are used in designing simplified sewers:
a =k_aD ................................................(4.8) r = k_rD ...............................................(4.9) The coefficients k_a and k_r are given from equations 4.8 and 4.9 as: