K.4 Design considerations

Once an appropriate sanitation and wastewater system has been identified, the infrastructure can be designed. This section provides guidance on the design of different collection, storage/treatment and conveyance infrastructure . It also deals with the calculation of sewage flow and design guidelines for waterborne sanitation systems, wastewater treatment infrastructure, greywater management systems and sludge disposal infrastructure. The section concludes with guidance on materials and the upgrading of various components of existing sanitation systems.

K.4.1 Design of collection, storage/treatment and conveyance infrastructure

The design of a sanitation facility should adhere to the relevant norms and standards as issued and updated by the DWS (refer to Section K.2.1). The design aspects of the most commonly used sanitation options are summarised below.

K.4.1.1 Ventilated Improved Pit toilet

Ventilated Improved Pit (VIP) and Ventilated Improved Double Pit (VIDP) toilets do not require water and thus fall within the dry toilet category. The design components are summarised in Table K.3 with typical design layouts illustrated in Figure K.5. For more information regarding the design, construction, operation and maintenance of a VIP toilet, consult Design, Construction, Operation and Maintenance of Ventilated Improved Pit Toilets in South Africa25, available from the WRC.

Table K.3: VIP toilets: Design aspects
Main component Sub - component Materials Design aspects
Substructure Pit In highly permeable soil (dry pit) OR In low permeable soil (wet pit) Can be circular or rectangular (circular more stable)
Not closer than 2.75 m from boundary fence (for maintenance purposes)
More than 30 m away and downhill from borehole/ well
Volume of Pit (m3)= P x N x C +0.5, where:
P = number of people (No)
N = design life (yr)
C = accumulation rate (m3/person /yr)
C (dry pits) = 0.06 m3/yr
C (wet pits) = 0.04 m3/yr
Pit lining Concrete blocks, openjointed brickwork, cement-stabilised soil blocks, masonry, stone rubble, perforated oil drums, rot-resistant timber, wire meshsupported geofabrics Only upper parts in stable soils (minimum 0.5 m from top)
Partial or full lining, depending on soil stability and groundwater presence
Top sections of pit walls shall be impervious to the passage of water
Stormwater and soil ingress to be prevented (lining to extend > 75 mm above ground level)

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Table K.3: VIP toilets: Design aspects
Main component Sub - component Materials Design aspects
Substructure Pit collar Reinforced concrete or bricks/ stone in cement mortar Must be sufficient to support cover slab
Slab Cover slab Reinforced concrete, ferrocement, bricks Must be properly supported Where a pit is without a collar, 200 mm wider than the pit On good support surface, 50 mm support is adequate
Reinforced slabs of 75 mm thickness with 6 mm bars at 150 c/c are adequate. 5:1 Sand/cement mix is sufficient. Keep slab damp for 5 days after pour.
Minimum of 75 mm above ground level
Maximum of 1 m above ground level
Holes for squat-hole and vent pipe to be formed
5% slope towards squat-hole for drainage
Superstructure General Materials depend on availability and affordability Ensure privacy, comfort and shelter
Waterproof and protect the user from the weather
Rectangular, circular or spiral shaped (no door required)
Movability of the structure should be considered when pit cannot be emptied
Floor Reinforced concrete Floor area = 0.8 to 1.5 m2 (2.35 m2 for VIDP)
At least 100 mm above general ground level to prevent flooding when it rains
Smooth for easy cleaning
Walls Brick and blocks preferred Ferrocement not advised Waterproof
Smooth for easy cleaning
Keep out disease-carrying vectors
Partially darkened structure preferred
Galvanised wire for vent pipe and roof to be provided
Door Wood, steel, composite materials (dependent on availability and affordability) Face the dwelling, depending on the preference of the users
Outward opening results in smaller inside area required
Inward opening decreases the risk of damage by wind
Lockable by key on the outside
Lockable by a catch on inside

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Table K.3: VIP toilets: Design aspects
Main component Sub - component Materials Design aspects
Superstructure Roof Reinforced concrete, corrugated iron, clay/ fibre cement tiles, thatch, palm leaves, etc. (dependent on availability and affordability) Waterproof
Tied to the walls to resist uplift forces
Slope away from the door
Seat Pedestal Brick, mortar, plastic, zinc, fibreglass, ceramic, wood, steel Beneficiaries to decide
Maximum width of slab opening of 200 mm
Seat opening 250 mm to 300 mm
Seat height 300 to 400 mm
A toilet seat and lid that can close
User needs are taken into account – kiddies seat, ramp for wheelchairs, etc.
Ventilation Ventilation pipe PVC, uPVC, bricks, blockwork, hessian (steel mesh-supported) etc. Painted black
Orientated towards the sun
Straight to attract flies upwards and maximise airflow
Preferably on outside of superstructure
Extending more than 500 mm above the highest point of the roof
At least 2 m away from anything that can impede airflow (trees, structures, etc.)
Ventilation openings   Provide without risking privacy
 > 3 times the area of ventilation pipe (0.15 m2 is adequate
Screened
Disease vector control Fly screen Corrosion resistant material (glass fibre, aluminium, stainless steel, brass, etc.) 1 mm to 1.5 mm mesh openings
Hand washing Basin/sink Brick, mortar, plastic, zinc, fibreglass, ceramic Running water within 1 m of the toilet
Water Potable/safe water
General considerations     Environmentally sound – protect and conserve water, energy efficient
Vulnerable groups (children, disabled, aged, women) are ensured safe access
Located to provide easy access for maintenance/ emptying

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Table K.3: VIP toilets: Design aspects
Main component Sub - component Materials Design aspects
General considerations     Downwind of dwelling, not nearer than 10 m and no further than 20 m
Orientated to ensure privacy and comply with cultural preferences (if applicable)
Appropriate solid waste disposal to be planned and designed for (including consideration of menstrual health needs)
Figure K.5: Examples of VIP (L) and VIDP (R) toilets
 

K.4.1.2 Composting toilet

The composting toilet is similar to the VIP toilet and the same design aspects for the superstructure and substructure need to be considered and incorporated. In a composting toilet, excreta fall into a tank/container to which ash or vegetable matter is added. The mixture will decompose to form a good soil conditioner in about four months. Pathogens are killed in the dry alkaline compost, which can be removed for application to the land as a fertiliser. Three types of composting toilets are presented below. In the traditional composting toilet (see Figure K.6), compost is produced continuously. With another type of composting toilet, the contents of the full pit is left to become compost. The pit is then used to plant a tree and a new toilet pit is dug, such as the Arborloo (see Figure K.7). A third composting toilet uses two containers to produce compost in batches, such as the Fossa Alterna (see Figure K.8). More information on composting toilets is available from the World Health Organization.27

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Figure K.6: Traditional composting toilet
Figure K.7: The Arborloo

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Figure K.8: Fossa Alterna toilet

K.4.1.3 Urine-diverting dry toilet

The urine-diverting dry toilet (UDDT) is a toilet that operates without water and has a divider/receptacle that diverts the urine away from the faeces. The faeces are dried out to be mixed with soil to form a compost. The urine can be collected in a container to be diluted with water and used as a soil conditioner, or it can be diverted to a soakaway. The design aspects for the superstructure of UDDTs are similar to those of the VIP toilet when it is not incorporated as part of the house.

The primary advantage of UDDTs, as compared to conventional dry toilets like VIP toilets, is the conversion of faeces into a dry odourless material. This leads to an odour-free and insect-free toilet that is appreciated by users and allows simple removal and less offensive and safer handling of the faecal material once the storage area has filled up. The functional design elements of a UDDT include source separation of urine and faeces, waterless operation and ventilated vaults or containers for faeces storage and treatment. Comprehensive design details can be obtained from Guidelines for the design, operation and maintenance of urine-diversion sanitation systems31 and from Technology Review of Urine-diverting dry toilets (UDDTs).32

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Figure K.9: Examples of a urine-diverting toilet (L) and a pour-flush sanitation system (R)
 

K.4.1.4 Pour-flush toilet

A pour-flush is a toilet fitted with a trap providing a water seal. It is cleared of faeces by pouring in sufficient quantities of water to wash the solids into the pit and replenish the water seal. The water seal prevents flies, mosquitos and odours reaching the toilet from the pit. The pit may be offset from the toilet by providing a short length of pipe or a covered channel from the pan to the pit. The pan of an offset pour-flush toilet is supported by the ground and the toilet may be within (or attached to) a house. The pour-flush can be retrofitted with a cistern to connect it to a waterborne sanitation system.

The design aspects for the superstructure of a pour-flush toilet are similar to those of the VIP toilet when it is not incorporated as part of the house. More details and information are available from the World Health Organization34 and from Developing a low flush latrine for application in public schools35, available from the WRC.

K.4.1.5 Aqua privy

An aqua privy is a toilet with the superstructure located directly above (or slightly offset of) a watertight holding tank. The tank is kept topped up with either potable water, rainwater, or greywater. The overflow of the tank can be connected to either a solids-free sewer system, or a soakaway. The design aspects for the superstructure are similar to those of the VIP toilet when it is not incorporated as part of the house. More details and information are available from A Guide to the Development of On-Site Sanitation.36

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Figure K.10: An example of an aqua privy
 

K.4.1.6 Septic leach field system

Septic tanks form part of the sewage disposal system that can be connected to the outlet of any water-flush latrine. An advantage of a septic tank is that the household has all the benefits of the conventional waterborne sanitation with on-site disposal. The disadvantage is that it requires the periodic removal of sludge.

A typical design of a septic tank is shown in Figure K.11. SANS 10252-2 Water Supply and Drainage for Buildings: Part 2 Drainage installations for buildings provides national standards on septic tank systems. Relevant information is included in Annexure B of SANS 10252-2.38

Figure K.11: A two-compartment septic tank with access risers and an effluent screen

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K.4.1.7 Anaerobic baffled reactor

An anaerobic baffled reactor is an improved septic tank. The retention time of the liquid in an anaerobic reactor is usually 30 to 50 days, which improves pathogen removal. The system can be connected to a solids-free system that removes the effluent for off-site disposal or to a soakaway, keeping the effluent on site and underground. Figure K.12 illustrates an anaerobic baffled reactor. More information is available from the Compendium of Sanitation Systems and Technologies.40

Figure K.12: An anaerobic baffled reactor (L) and an anaerobic digester (R)
 

K.4.1.8 Anaerobic digester/ Biogas reactor

An anaerobic digester is an airtight container in which the waste is dumped and decomposed. Bacteria within the digester tank breaks down the waste and, as it decomposes, gases such as carbon monoxide, methane, hydrogen, and nitrogen, are released. The gas, known as biogas, is captured in a gas holder to be used later to be combusted, or reacted, with oxygen to create an energy source for such processes as heating and vehicle propulsion.

Refer to SANS 1753 The Construction, installation, commissioning and maintenance of any biogas plan, piping, controls and equipment42 for the relevant standards pertaining to anaerobic digesters and biogas. More information is available from Decentralised Wastewater Treatment Systems (DEWATS) and Sanitation in Developing Countries. A Practical Guide43 and the Compendium of Sanitation Systems and Technologies.44

K.4.1.9 Vacuum sewer system

Vacuum sewer systems make use of a combination of gravity and differential air pressure as the driving force that propels sewage through the sewer network. Vacuum sewer systems consist of three key components: collective chambers, vacuum sewers and the vacuum station. A central vacuum pump station is required to maintain a vacuum (negative pressure) on the collection system (see Figure K.13). The system requires a normally closed vacuumgravity interface valve at each entry point to seal the lines so that the vacuum can be maintained.45 These valves, located in valve pits, open when a predetermined amount of wastewater accumulates in collecting sumps.

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The resulting differential pressure between the atmosphere and vacuum becomes the driving force that propels the wastewater towards the vacuum station. For design details refer to the Waterborne Sanitation Design Guide46 and the City of Cape Town’s Service Guidelines and Standards for the Water and Sanitation Department.47

Figure K.13: Vacuum sewer system
 

Small-bore systems, or small-diameter-gravity (SDG) sewers, or solids-free sewers (SFS) are also called septictank-effluent-gravity (STEG) sewers. These systems convey effluent by gravity from an interceptor tank (or septic tank) to a centralised treatment plant or pump station, from where it is conveyed to another collection system. Another variation on this alternative sewer system is the septic-tank-effluent-pumping (STEP) concept. All these systems utilise smaller-diameter pipes placed in shallow trenches that follow the natural contours of the area, thus reducing the capital cost of the pipe, as well as excavation and construction costs. For design detail, refer to the Waterborne Sanitation Design Guide49 published by the WRC.

K.4.1.10 Small-bore sewer system

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Figure K.14: Small-bore sewer system
 

K.4.1.11 Simplified/shallow sewer system

A simplified sewer system is constructed using smaller diameter pipes laid at a shallower depth and at a flatter gradient than conventional sewers in order to remove wastewater from the household environment. Many of the conventional sewer design standards, such as minimum diameter, minimum slopes and minimum depths are relaxed in shallow sewer systems, and community-based construction, operation and maintenance are allowed. Expensive manholes are replaced by simple inspection chambers. Each discharge point is connected to an inspection and/ or baffle to prevent solids and trash from entering the system. Another key design feature is that the sewers are laid within the property boundaries rather than beneath central roads. Since the sewers are considered more ‘communal’, they are often referred to as ‘condominial sewers’51. Simplified sewer systems can be installed in almost all types of settlements and are especially appropriate for dense urban settlements. For design detail, refer to the Waterborne Sanitation Design Guide52 published by the WRC.

K.4.1.12 Waterborne sanitation

Waterborne sanitation consists of a flush toilet connected to reticulation that transports sewage away from the user. Section K.4.2, Section K.4.3 and Section K.4.4 apply to the design of waterborne sewerage reticulation. Certain basic guidelines applicable to non-gravity systems (i.e. pump stations and rising mains) are included, but detailed design criteria for these systems are not included, as they are regarded as bulk services. Except in cases where illustrations are provided, the reader is referred to figures in the relevant sections of SANS 1200 Standardised Specification for Civil Engineering Construction.53

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