Waste water infrastructure design reports are essential documents in civil engineering projects, especially in Kenya where urbanisation and industrial growth demand efficient sanitation systems. These reports detail the design elements, construction appurtenances, and operational considerations necessary for sustainable waste water management. Understanding the components like valves, fire hydrants, meters, chambers, meter chambers, and surge tanks is critical to preparing comprehensive reports that guide construction and maintenance phases effectively.

5.1 Construction Appurtenances

Construction appurtenances are auxiliary components integrated into waste water infrastructure to ensure operational control, safety, and monitoring of the system. In Kenya’s civil engineering practice, appurtenances are vital for managing flow, preventing contamination, and facilitating maintenance in municipal and industrial waste water networks. This section explores key appurtenances commonly specified in design reports to support system functionality and durability.

5.1.1 Valves

Valves control the flow and pressure within waste water pipelines, providing essential regulation and isolation capabilities. Their correct specification and placement in the system prevent backflow, enable repairs without system shutdown, and protect infrastructure from pressure surges.

Materials and Specifications

• Valves in waste water systems are commonly fabricated from cast iron, ductile iron, stainless steel, or PVC, depending on the chemical composition of the waste water and exposure conditions.
• Material selection considers corrosion resistance, mechanical strength, and cost-effectiveness to ensure longevity in Kenya’s diverse environments.
• For example, a valve used in a tannery waste water system near Nairobi must resist acidic and chemical corrosion, often necessitating stainless steel with special coatings.

Installation Considerations

• Valves must be installed with adequate access for operation and maintenance, often in valve chambers or pits.
• Proper alignment and anchoring prevent mechanical stress on pipelines during operation.
• In Nairobi County’s sewer networks, valves are strategically located at junctions, pump stations, and treatment plant inlets for operational flexibility.

Maintenance and Testing

• Routine inspection includes checking for leakage, corrosion, and operability.
• Valve actuators and seals require periodic lubrication and replacement to maintain functionality.
• Pressure testing after installation ensures valves meet design specifications and operate safely under system pressures.

5.1.1.1 Gate

• Gate Valves: Used primarily for on/off control, gate valves allow full flow or complete shutoff with minimal pressure drop, making them suitable for large diameter pipelines in municipal waste water networks.

Large industrial gate valve outdoors (Source: Wikimedia Commons)

A gate valve closes off flow by lowering a flat or wedge-shaped gate across the full bore of the pipe, operated by a handwheel or key turning a threaded stem. When fully open, the gate is withdrawn completely from the flow path, so solids in waste water pass freely and head loss is very low, which makes gate valves the usual choice for isolating sewage rising mains, pump stations and treatment units. For sewage, resilient-seated or knife gate valves are preferred, because a knife-edged gate cuts through rags and solids that would stop a wedge from sealing. Example: the pump station serving a county hospital’s sewage rising main had knife gate valves on each pump outlet so that one pump could be removed for repair while the other kept running.

Guidelines for Gate Valves in Waste Water Systems

• Use for isolation only: Operate fully open or fully closed, since partly open gates trap solids and erode.
• Choose knife gates for solids: Knife gate valves cut through fibrous material and seal reliably in raw sewage.
• Close slowly: Close large valves gradually to avoid water hammer in rising mains.
• Exercise regularly: Open and close valves periodically so grease and solids do not seize them.
• Provide access: Install valves in chambers with space to operate and remove them.

5.1.1.2 Globe

A globe valve regulates flow by moving a disc or plug towards or away from a seat inside a rounded body, with the liquid changing direction as it passes through. It gives fine and accurate flow control and a tight seal, but its narrow, winding passage causes high head loss and traps solids, so it is not used on raw sewage lines. In waste water works, globe valves are used on clean or treated liquid lines such as chemical dosing, sampling, seal water and compressed air lines, and on treated effluent lines where flow must be set accurately. Example: at the treatment plant of a boarding school, small globe valves were fitted on the chlorine dosing line so that the operator could set the disinfectant dose precisely.

Gate and Globe Valves Compared

5.1.1.3 Check

• Check Valves: Designed to prevent backflow, check valves protect pumping equipment and pipelines from damage caused by reverse flow, essential in systems with fluctuating pressures.

A check valve (non-return valve) lets waste water flow in one direction only and closes automatically when flow tries to reverse. On a sewage pumping main, it stops the column of sewage in the rising main from draining back through the pump when the pump stops, which would otherwise spin the pump backwards and flood the wet well. It is also used at outfalls and on connections where backflow could cause flooding or contamination. Example: at a hotel’s sewage pump station, a ball check valve on each pump discharge prevented sewage from returning to the sump every time the pumps switched off.

Types of Check Valves for Waste Water

• Ball check valve: A rubber-coated ball rolls off the seat with flow and back onto it when flow stops; it has a clear bore and suits raw sewage.
• Swing check valve: A hinged flap swings open with flow; models with an external lever and weight are used on larger sewage mains.
• Duckbill (flexible) check valve: A flexible rubber sleeve opens with flow and closes against back pressure; it is used at outfalls to keep out river or tidal water.
• Flap valve: A hinged flap at a pipe outlet prevents water from entering drains during floods.
• Dual-plate check valve: Spring-loaded plates close quickly; it suits treated effluent rather than raw sewage.

5.1.1.4 Air relief

• Air Release Valves: Installed at high points in pipelines, air release valves expel trapped air, preventing air locks that can reduce flow efficiency and cause pipeline damage.

Operation of air release valve stages (Source: Wikimedia Commons)

Air relief valves remove air and gases from pressurised waste water pipelines and, in some types, let air in to prevent vacuum. Sewage rising mains collect air and also sewage gases such as hydrogen sulphide at high points, which reduce flow capacity, cause surges and corrode the pipe crown. Valves made for sewage have a longer body and special float mechanism so that solids and grease do not reach and block the air outlet, and they are installed at high points and changes of gradient with an isolating valve below for maintenance. Example: the rising main from an abattoir in Athi River lost capacity until sewage air valves were fitted at its two high points, after which the pumps again delivered their design flow.

Types of Air Valves

5.1.1.5 Pressure reducing

A pressure reducing valve (PRV) automatically lowers a high, varying upstream pressure to a lower, steady downstream pressure. In waste water systems, PRVs are used mainly on pressurised clean or treated lines, such as treated effluent reuse lines, wash water and service water lines, and on sewage pressure mains where a long downhill section would otherwise create excessive pressure at the low end. A PRV senses the downstream pressure and opens or closes its main valve to keep that pressure at the set value, and it is installed with isolating valves, a strainer, pressure gauges and a bypass. Example: a flower farm in Naivasha reusing treated effluent for irrigation fitted a PRV where its pipeline dropped steeply into the valley, protecting the drip lines from bursting.

Worked Example: Pressure at the Foot of a Steep Pipeline

A treated effluent pipeline falls 90 m from a storage tank to an irrigation block. Find the static pressure at the bottom, and state the effect of a PRV set at 3 bar.

p = ρ × g × h

p = 1,000 × 9.81 × 90

p = 882,900 Pa = 8.8 bar

A PRV set at 3 bar reduces the pressure supplied to the irrigation block from about 8.8 bar to 3 bar.

Steps for Installing a Pressure Reducing Valve Assembly

1. Select the valve: Choose a PRV rated for the pressure, flow and liquid quality.
2. Construct the chamber: Build a chamber large enough for the valve, fittings and access.
3. Fit the upstream isolating valve and strainer: Protect the PRV from debris and allow isolation.
4. Install the PRV: Fit it in the correct flow direction, supported independently of the pipe.
5. Fit pressure gauges: Install gauges upstream and downstream to monitor performance.
6. Install the downstream isolating valve and bypass: Allow maintenance without stopping flow.
7. Set and test: Adjust the outlet pressure and check its response at different flows.

5.1.1.6 Butterfly Valves

• Butterfly Valves: These valves are compact and lightweight, offering quick operation for regulating flow. Their ability to throttle flow makes them suitable for treatment plants and pumping stations.

5.1.1.7 Pressure Relief Valves

• Pressure Relief Valves: These valves protect pipelines from excessive pressure by releasing fluid when preset pressure limits are exceeded, preventing bursts and system failure.

5.1.2 Fire hydrants

Fire hydrants in waste water infrastructure serve dual roles: providing access to water for firefighting and facilitating system flushing and maintenance. Their integration into sewer systems or alongside pipelines supports emergency response and operational hygiene.

Purpose and Location

• Fire hydrants are typically located at strategic intervals along pipeline routes, near industrial estates, hospitals, schools, and residential estates to ensure quick fire response.
• In urban areas like Mombasa, hydrants also enable flushing of sewer lines to prevent blockages and accumulation of solids.
• Hydrants must be accessible and visible, complying with Kenya’s National Fire Safety Guidelines.

Design Specifications

• Hydrants must withstand high pressure and environmental exposure.
• Anti-corrosion coatings are essential in coastal areas such as Kilifi County due to saline conditions.

Operation and Maintenance

• Hydrants require regular functional testing to confirm water flow and pressure adequacy.
• Lubrication of operating nuts and replacement of gaskets prevent leaks.
• In Kisumu County, hydrants are flushed quarterly to maintain cleanliness and prevent sediment buildup.

5.1.2.1 Wet barrel

• Wet Barrel Hydrants: Common in warmer climates, these hydrants maintain water in the barrel, allowing immediate water flow upon operation.

A wet barrel hydrant has its barrel (upright body) permanently full of water under mains pressure, with an individual valve behind each outlet. Because water is already in the barrel, each outlet can be opened independently and gives water immediately, which allows fire crews, or sewer maintenance crews using water for flushing, to connect quickly. Wet barrel hydrants suit Kenya’s generally frost-free climate, but they need protection from vehicle impact because damage to the barrel releases water. Example: a business park in Kisumu used wet barrel hydrants along its roads, and the same hydrants supplied water to jetting units that cleared its sewer lines.

Features of Wet Barrel Hydrants

• Barrel always charged: Water stands in the barrel at mains pressure.
• Separate outlet valves: Each outlet has its own valve and can be used independently.
• Immediate supply: Water flows as soon as an outlet valve is opened.
• Suited to warm climates: There is no risk of freezing damage.
• Needs impact protection: Bollards are often provided in traffic areas.

5.1.2.2 Dry barrel

• Dry Barrel Hydrants: Used in areas prone to freezing, the barrel drains after use to prevent damage.

Dry barrel hydrant sectional diagram (Source: Wikimedia Commons)

A dry barrel hydrant has its main valve below ground at the base, so the barrel above ground is empty when the hydrant is closed and drains through a drain hole after use. Turning the operating nut on top opens the main valve, filling the barrel and supplying all outlets together. This design prevents freezing and limits water loss and contamination risk if the hydrant is damaged, which is useful at high altitude and in places exposed to vehicles or vandalism. Example: a county water department supplying a town on the slopes of Mount Kenya fitted dry barrel hydrants at exposed roadside sites.

Steps for Operating a Dry Barrel Hydrant

1. Remove the outlet cap: Remove the cap from the outlet to be used and check the thread.
2. Connect the hose: Connect the hose or flushing unit securely.
3. Open the main valve fully: Turn the operating nut slowly until fully open so the drain hole closes.
4. Use the water: Supply the fire appliance or flushing equipment.
5. Close slowly: Turn the nut slowly to close and avoid water hammer.
6. Drain the barrel: Leave the outlet open briefly so the barrel empties through the drain.
7. Replace the cap and report faults: Refit the cap and record any defects.

5.1.2.3 Subsurface Hydrants

• Subsurface Hydrants: Installed underground with a valve operated by a wrench, often used in congested urban settings for aesthetic and safety reasons.

5.1.3 Meters

Meters in waste water infrastructure measure flow rates and volumes, providing critical data for system monitoring, billing, and leak detection. Accurate metering supports operational efficiency and resource management in municipal and industrial settings.

Selection Criteria

• Selection depends on pipe diameter, flow range, fluid properties, and maintenance requirements.
• In Nairobi’s industrial zones, electromagnetic meters are preferred for their durability and minimal maintenance in contaminated flows.

Installation Guidelines

• Meters must be installed in straight pipe sections free from turbulence to ensure accuracy.
• Proper grounding and shielding are necessary for electromagnetic meters to function correctly.
• Accessibility for calibration and maintenance is crucial, often requiring dedicated meter chambers.

Data Utilization

• Flow data assists in detecting leaks, evaluating treatment plant performance, and billing for industrial effluent discharge.
• County governments use meter data to enforce compliance with waste water discharge permits under NEMA regulations.

5.1.3.1 Domestic

Domestic meters are small water meters, usually 15 mm to 25 mm, fitted on household and small business water connections. Waste water from homes is rarely metered directly, so utilities use the water consumed, read from the domestic meter, to calculate sewerage charges, commonly assuming that a fixed proportion, often around 75% to 80%, returns to the sewer. Domestic meter records also help engineers estimate per capita water use and therefore the design flows for sewers. Example: a water and sewerage company billing an estate in Nakuru calculated each household’s sewer charge as a percentage of its metered water bill.

Worked Example: Sewerage Charge from a Domestic Meter

A household’s meter reading rose from 1,245 m³ to 1,263 m³ in a month. The utility charges sewerage at 75% of the water consumed at Ksh 60 per m³. Find the sewerage charge.

Water used = 1,263 − 1,245 = 18 m³

Sewage volume = 0.75 × 18 = 13.5 m³

Sewerage charge = 13.5 × 60

Sewerage charge = Ksh 810

Features of Domestic Meters

• Small size: Typically 15 mm to 25 mm to suit household connections.
• Accuracy at low flows: Records small flows such as dripping taps.
• Positive displacement or multi-jet design: Mechanical measuring elements suited to clean water.
• Tamper resistance: Seals and protected registers prevent interference.
• Easy reading: Clear registers, often with remote reading options.

5.1.3.2 Bulk

Bulk meters are large meters, generally 50 mm and above, that measure large flows at strategic points in water and waste water systems. In waste water infrastructure they are installed on treatment works inlets and outlets, pumping station rising mains and the discharge points of large industrial customers, so that operators know how much waste water is received, treated and discharged. Because sewage contains solids, bulk meters for waste water are usually electromagnetic or ultrasonic, or open-channel devices such as flumes, rather than mechanical meters. Example: a brewery discharging effluent into a municipal sewer had a bulk electromagnetic meter on its outlet so that the county could bill its discharge by volume.

Uses of Bulk Meters in Waste Water Systems

• Treatment works monitoring: Measures inflow to check whether the plant is overloaded.
• Industrial effluent billing: Measures the volume discharged by factories for charging.
• Pump performance checking: Shows whether pumps deliver their design flow.
• Infiltration detection: Rising night flows show groundwater or stormwater entering sewers.
• Permit compliance: Provides discharge records required by NEMA and county authorities.

5.1.3.3 electromagnetic

• Electromagnetic Flow Meters: Ideal for waste water due to no moving parts and resistance to clogging by solids.

Electromagnetic flow meter diagram (Source: Wikimedia Commons)

An electromagnetic flow meter works on Faraday’s law of electromagnetic induction: as waste water, which conducts electricity, flows through a magnetic field set up by coils around the pipe, it generates a small voltage proportional to its velocity, which electrodes in the pipe wall measure and convert to a flow rate. The meter has a full, unobstructed bore and no moving parts, so solids, rags and grit pass through without blocking or wearing it, and it causes no extra head loss. It needs a power supply and must run with the pipe completely full to read correctly. Example: the inlet of a university’s sewage treatment plant was fitted with an electromagnetic meter, giving continuous flow records that showed when infiltration increased during the rains.

Steps for Installing an Electromagnetic Meter

1. Select the location: Choose a point where the pipe always runs full, such as a low point or rising section.
2. Provide straight lengths: Allow the straight pipe lengths upstream and downstream specified by the manufacturer.
3. Build the chamber: Construct a drained, accessible chamber with room for the meter and converter.
4. Install the meter: Fit the meter between flanges with the flow arrow in the correct direction.
5. Earth the meter: Fit earthing rings and connections as specified to avoid electrical interference.
6. Connect power and signals: Wire the converter to power and to the data logger or SCADA system.
7. Configure and commission: Enter pipe size and units, then check readings against a reference.

5.1.3.4 Ultrasonic Flow Meters

• Ultrasonic Flow Meters: Employ sound waves to measure flow velocity, suitable for large diameter pipes.

Industrial ultrasonic flow meter installed on pipeline (Source: Wikimedia Commons)

5.1.3.5 Turbine Flow Meters

• Turbine Flow Meters: Utilize a rotor to measure flow, often applied in clean water but less common in waste water due to clogging risk.

5.1.3.6 Venturi Meters

• Venturi Meters: Measure differential pressure to calculate flow, effective for consistent flow regimes.

5.1.4 Chambers

Chambers are structural enclosures that provide access to underground pipelines and appurtenances for inspection, maintenance, and flow regulation. Their design must consider safety, accessibility, and hydraulic efficiency.

Design Considerations

• Chambers must be sized to allow safe entry and movement of personnel, typically minimum internal diameters of 900 mm.
• Structural materials include reinforced concrete and precast concrete rings, selected for durability and ease of installation.
• In Kisii County, chambers incorporate corrosion-resistant coatings due to acidic soil conditions.

Safety Features

• Chambers require secure covers to prevent unauthorized access and accidents.
• Ventilation openings prevent accumulation of hazardous gases.
• Lighting and ladders are necessary for deep manholes to facilitate safe inspection.

Maintenance and Inspection

• Regular inspection involves checking for blockages, structural integrity, and signs of infiltration or exfiltration.
• Cleaning schedules prevent sediment buildup, ensuring smooth flow and reducing maintenance costs.

5.1.4.1 Meter

A meter chamber is an enclosure built to house a flow meter and its fittings, protecting it from damage, flooding and tampering while allowing it to be read, calibrated and replaced. In waste water systems, meter chambers are built on rising mains, treatment works inlets and outlets, and industrial effluent discharge points, and they must hold the meter, isolating valves, a dismantling joint for removal and, for electromagnetic meters, the converter and cable entries. Because the chamber may contain sewage gases, it must be ventilated and treated as a confined space for entry. Example: a slaughterhouse connected to a municipal sewer built a meter chamber at its boundary, so that county staff could read and check the effluent meter without entering the factory.

Steps for Constructing a Meter Chamber

1. Set out the chamber: Mark the position and internal size to suit the meter assembly and working space.
2. Excavate and prepare the base: Excavate to depth and compact the formation.
3. Cast the base slab with a sump: Provide a sump so water can be pumped out.
4. Build the walls: Construct reinforced concrete or masonry walls with sleeves for the pipes and cables.
5. Install the meter assembly: Fit isolating valves, the meter and a dismantling joint on supports.
6. Seal openings and provide ventilation: Seal pipe sleeves and fit vents to disperse gases.
7. Fit the roof slab and cover: Provide a lockable, load-rated access cover and ladder or step irons.
8. Backfill and label: Backfill around the chamber and mark it clearly.

5.1.4.2 valve

A valve chamber is an underground enclosure that houses valves such as gate valves, check valves, air valves or washouts, so they can be operated, inspected and maintained. On sewage rising mains and in pump stations, valve chambers must be large enough to operate and remove the valves, have a sump or drain for spillage, and be ventilated because sewage gases are toxic and flammable. The pipe should pass through the chamber walls on flexible seals so that settlement of the chamber does not crack the pipe. Example: each air valve on the rising main from a cooperative coffee factory’s effluent pump station was housed in a ventilated concrete valve chamber with a lockable cover.

Requirements for Valve Chambers

• Adequate size: Space to turn the valve and remove it for repair.
• Independent pipe support: Pipes pass through the walls on flexible seals and are supported separately.
• Drainage: A sump or drain prevents sewage or water from collecting around the valve.
• Ventilation: Vents disperse hydrogen sulphide and methane.
• Safe access and identification: Ladders or step irons, secure covers and clear labels are provided.

5.1.4.3 Manholes

• Manholes: Vertical access points for personnel entry, inspection, and maintenance.

A manhole cover on a concrete surface (Source: Wikimedia Commons)

5.1.4.4 Junction Chambers

• Junction Chambers: Where multiple pipelines converge or diverge, facilitating flow control and monitoring.

5.1.4.5 Drop Chambers

• Drop Chambers: Designed to manage changes in pipeline elevation, preventing turbulence and sediment deposition.

5.1.4.6 Inspection Chambers

• Inspection Chambers: Smaller than manholes, used for visual inspection without personnel entry.

5.1.5 Meter chambers

Meter chambers are specialized enclosures designed to house flow meters and associated instrumentation, protecting them from environmental damage and facilitating accurate flow measurement.

Purpose and Importance

• Meter chambers safeguard sensitive metering equipment from vandalism, weather, and mechanical damage.
• They provide a controlled environment for calibration and maintenance without disrupting the waste water flow.
• In Nairobi’s County water and sanitation department, meter chambers improve data reliability for billing and system management.

Design Features

• Chambers should be watertight to prevent infiltration of stormwater, which can skew meter readings.
• Adequate space is required for meter installation, wiring, and operator access.
• Materials used include reinforced concrete or durable plastics resistant to chemical corrosion.

Installation and Accessibility

• Located at accessible points along the pipeline, often adjacent to valve chambers.
• Must include provisions for cable entry, ventilation, and sealing to maintain integrity.
• Design must comply with Kenya’s Occupational Safety and Health Act to ensure operator safety during maintenance.

Maintenance Protocols

• Regular calibration and cleaning of meters within the chamber maintain measurement accuracy.
• Chambers are inspected for structural damage, water ingress, and security breaches.
• Proper drainage within the chamber prevents water accumulation that could damage electronic components.

5.1.6 Surge tanks

Surge tanks are protective appurtenances used to mitigate pressure fluctuations in waste water systems, preventing pipeline damage caused by water hammer and sudden changes in flow velocity.

Function and Importance

• Surge tanks absorb pressure surges caused by pump starts/stops or valve closures, stabilizing hydraulic conditions.
• They prevent pipe bursts and joint failures, reducing costly repairs and service interruptions.
• For instance, in large pumping systems serving Nairobi’s industrial estates, surge tanks protect pipelines from transient pressures.

Types of Surge Tanks

• Open Surge Tanks: Open to atmosphere, these tanks allow water level variations to absorb pressure changes.

Open surge tank with pipeline in a trench (Source: Wikimedia Commons)

• Closed Surge Tanks: Sealed tanks using air cushions or gas to dampen pressure fluctuations.
• Standpipe Surge Tanks: Vertical pipes connected to the pipeline, providing surge relief through water column movement.

standpipe surge tank with pipeline (Source: Wikipedia (Surge tank))

Design Parameters

• Surge tank sizing depends on pipeline diameter, flow velocity, and pump characteristics.
• Design must account for Kenya’s seismic zones to ensure structural resilience.

Worked Example: Surge Pressure from Pump Trip

Sewage flows at 2.0 m/s in a rising main when the pump suddenly stops. Taking the pressure wave speed as 1,100 m/s, estimate the pressure change.

Δp = ρ × c × Δv

Δp = 1,000 × 1,100 × 2.0

Δp = 2,200,000 Pa = 22 bar

A surge of this size could burst the main or collapse it under vacuum, which is why surge tanks or surge vessels are provided on long rising mains.

Installation and Operation

• Surge tanks are installed near pump stations or at high points in the pipeline.
• Regular inspection includes checking for sediment accumulation and structural integrity.
• Proper operation requires coordination with control systems to avoid excessive water level fluctuations.

Chapter Summary

This chapter provides a comprehensive overview of preparing a wastewater infrastructure design report, beginning with an examination of construction appurtenances such as various types of valves including gate, globe, check, air relief, and pressure reducing valves, as well as fire hydrants categorized into wet barrel and dry barrel types. It also covers meters, distinguishing between domestic, bulk, and electromagnetic varieties, along with chambers for meters and valves, and the role of surge tanks in the system. The chapter then addresses the preparation of technical specifications that define the standards and requirements for the infrastructure components. Following this, the bill of quantities is explained, highlighting the importance of accurate measurements and cost estimations to ensure practical budgeting and resource allocation. The final section focuses on the technical design report itself, detailing the design methodology, specific design details, system components, and a thorough cost analysis that includes item descriptions, quantities, and recommended trainee ratios. Together, these elements form the essential framework for documenting and executing effective wastewater infrastructure projects.