Method Statement for Flushing and Disinfection of Potable Water Network

method statement for flushing and disinfection of potable water network


This method statement for flushing and disinfection of potable water network defines the required procedure for potable water main flushing, testing, disinfection, and de-chlorination prior to placing the network in service.

This method ensures that all works are carried out in a safe manner in accordance with the contract specifications and approved shop drawings.


Flushing & Disinfection of Potable Water Network for the satisfaction and approval of the consultants and the client. The work method was prepared for the complete execution of the ‘’Flushing & Disinfection of Potable Water Network’ for the project.


This method statement is subject to the requirements as stated in the following regulatory & statutory documents, applicable standards, specifications, and building codes:
Project Specifications
Relevant Sections of IPC & Local authorities
Project approved IFC drawings & materials
Approved Project HSE Plan
Approved Project Emergency Response Plan
Approved Project Waste Management Plan
Occupational Health and Safety (Regulatory Document)


MS – Method Statement
QCP – Quality Control Procedures
ITP – Inspection and Test Plan
ITA – Independent Testing Agency
MEP – Mechanical, Electrical & Plumbing
HSE – Health, Safety & Environment


Works will be started or carried out only when all associated main contractors procedures are completed and by ensuring safe access to work areas will be available through hoist lifts/access ladders to carry out the work where ever main contractor site engineer’s & safety officers clearance available.


MEP Subcontractor will deploy a dedicated team for the required logistics and material movement in -out of the project.

Health & Safety Hazards

Extreme heat and humidity

Extreme cold, high winds, and dusty condition

Mobile equipment- crane, forklift, etc. for material loading/offloading, transfer of materials to work locations.

Temporary electrical supply for lighting, use of power tools, etc.

Improper stacking of materials

Manual handling/offloading & installation of materials

Working on the unsafe scaffold during installation activity

Damaged hand tools

Unsafe working on ladders.

Poor housekeeping & waste management.

MEP subcontractor will comply with all safety measures as applicable to the project’s main safety requirements and follow approved HSE plans.

Information to personnel regarding health and safety:

HSE Site Safety Inductions
Ensure STARTT and/or RAMS are discussed with the work group prior to works commencing
Safety toolbox talks
Environmental toolbox talks
HSE training records / Toolbox attendance register
Training courses ( Internal and Third-party Training )
HSE Bulletin/ HSE Site Notice Boards
Site Engineers, Site Supervisors, and Site Foreman

Permits and Certificates:

HSE Permits and Certifications (as per activity requirements) – These should correlate with the activity Risk Assessment.

Permit To Work (Prior to starting an activity):

Ensure obtain the necessary PTW (Permit to Work), prior to allowing to work. Frequently training should be conducted by the site supervisor/foreman.

Hot work permit

Lifting Plan

Working at heights/working on scaffolding or MEWP (Mobile Elevating Work Platform) Third-party certification for workers (competency certificates), operators, and equipment.


Project Manager

The project manager shall be responsible for overall project planning, procurement, technical submissions, and drawing submissions.

The project manager shall coordinate activities within the site team and with the management for the timely completion of activity in accordance with contract requirements.

Construction Engineer

The construction manager is responsible for the work execution in compliance with the approved method statement, HSE Risk Assessment, and project specification, issued for construction drawings, sections, and details

Project Engineer

The Project engineer for each discipline shall be responsible for the execution of the complete system in accordance with project requirements. He will be responsible for the day-to-day activities on site, for materials, drawings, testing & commissioning, and allocation of departmental resources to work areas on site. The project engineer will report to the MEP project manager.

The project engineer shall coordinate all related MEP system activities with all other trades for progress monitoring.

Site Engineer

The site engineer is responsible to carry out the work as per approved shop drawings and method statements. He is also responsible to ensure adequate tool kits required for the works are in place prior to the starting of work. The site engineer reports to the construction manager.

The site engineer is responsible to ensure work has been done as per the specification and drawings; inspections are carried out based on ITP and quality control procedures and coordinated with QA/QC department for inspections.

The site engineer is responsible to ensure that all safety precautions are ensured in place and that all personnel on site are using the required personal protective equipment like helmets, gloves, shoes, and reflective jackets. The site engineer in coordination with safety officers will ensure all relevant permissions are taken prior to the commencement of works.

QA/QC Engineer

QA/QC engineer will be responsible for overseeing the quality requirements and quality control measures for the project in coordination with the project engineer and construction manager.

Safety Engineer

The safety engineer will be responsible for the safety, health, and environmental aspects on site as per the project safety and environmental requirements/safety standards in coordination with the project engineer and construction manager.


Material Handling & Storage

On receipt of pipes and fittings, the materials shall be handled and stored in line with the following procedure:

All materials delivered to the site shall be new, free from defects, and of the same type & model as per the approved material submittal.

The materials should be received factory packed and should remain so until installation.

Delivered materials shall be carefully off-loaded using necessary manpower, or lifting equipment as necessary to ensure no damage occurs. All lifting and handling procedures will conform to the manufacturer’s recommendations.

Upon completion of receiving inspection, the chemicals are to be stored in a designated area for chemical storage.

Materials are to be stored on a flat surface in the well-ventilated, dry, and covered storage area.

Materials shall be fully protected/covered at all times.

Procedure for Domestic Water Tank Disinfection / Sterilization

This procedure describes methods of disinfecting water-storage facilities that are newly constructed, have been entered for construction or inspection purposes, or that continue to show the presence of coliform bacteria during normal operation.

In addition, the standard defines disinfection procedures for underwater inspection because water utilities increasingly are employing divers to conduct underwater inspections of on-line potable water storage facilities to minimize water loss and downtime normally associated with necessary maintenance inspections.

potable water system setup

1. Cleaning of the tank

All scaffolding, planks, rags, and other materials not part of the structure or operating facilities of the tank shall be removed.

Then the surfaces of the walls, floor, and operating facilities of the storage facility shall be cleaned thoroughly using a high-pressure water jet, sweeping, scrubbing, or equally effective means.

All water, dirt and foreign material accumulated in this cleaning operation shall be discharged from the storage facility or otherwise removed.

Following the cleaning operation, the vent screen. Overflow and any other screened openings shall be checked and put in satisfactory condition to prevent birds, insects, and other possible contaminants from entering the facility.

Any material required to be in the operating storage facility after the cleaning procedure has been completed shall be clean and sanitary when placed in the facility.

In such instances, care shall be taken to minimize the introduction of dirt or other foreign material.
(For example, placing a layer of limestone granules on the unpainted bottom of the storage facility to prevent corrosion.

2. Chlorinate or disinfect the tank

To effectively disinfect the tank, fill it with clean water up to 25% of tank level only. It is important to not fill the tank too much as this will reduce the concentration of the chlorine solution and limit the effectiveness of cleaning.

To estimate ¼ of the tank, use graded markers to indicate the water level. The markers should be marked onto the stick at 10cm intervals beginning at 0cm at the base of the tank and then upwards to 10, 20cm, etc.

Prepare a concentrated chlorine solution to disinfect the tank. The best source of chorine to use is high test calcium hypochlorite granules or powder form as this normally contains 60-70% chlorine.

Pour the solution slowly into the tank, mixing as you pour, and then fill the tank up to full capacity with clean water.

Let the chlorine stand in the tank for 6 hours to ensure the tank is fully disinfected. If the tank has a cover it is recommended to be closed.

If the tank is required for use urgently double the quantity of chlorine added to the tank.

This will reduce the standing time from 6 to 8 hours.

Remember to also clean and disinfect any pipe or hoses connected to the tank

3. Disposal of chlorinated water and final flushing

After 24 hours of retention time, chlorinated water shall be disposed of carefully as per the below procedures.
Disposal of disinfected water shall conform to all applicable standards or international standards such as WHO/AWWA.

Check with the local sewer department for conditions of disposal to sanitary sewer and with the state regulatory agency for conditions of disposal to natural drainage courses.

Drain the chlorinated water using a drain submersible pump or by opening the drain valve until the tank reaches empty.

Ensure the tank is empty and keep the tank idle for an hour time.

Now again flush the tank with fresh water until the chlorine odor reduces. Use dechlorinating or SMBS chemicals to reduce the concentration of chlorine.

Further, refill the tank with fresh water and allow it to stand for 30mins.

4. Sampling and testing

After refilling the tank, obtain samples from a different location in PE sterilized bottles and send them to a third laboratory.

Also, you can check the residual chlorine concentration using a potable regent testing kit. It is important to check the below parameters after the cleaning activity.

  • Total coliform
  • Free residual chlorine.

Disinfection of Mains Supplies & Newly Laid Main Pipeworks

1. General Disinfection

The following disinfection procedure is proposed to remove debris such as silt, sand, weld slug and other building/ plumbing materials, bacterial new mains pipe work, s refurbished or altered mains fed system or a mains system that has produced a high bacterial count. If systems are heavily contaminated or show the presence of biofilm then additional methods may be required to remove this from pipe work internals.

It is important to ensure that when chlorinating mains water systems that all precautions are taken to ensure no backflow of chlorinated water in the mains supply. It is also important to ensure that people working within the area fed by the water system are informed and cannot accidentally chemically treated water.

2. Disinfection Procedure

Sign into site and obtain necessary permits to work
Perform a full risk assessment of the area around the system to be disinfected and note pipe diameter, and length, and produce a rough diagram of the system.

Before any disinfection procedure begins place warning boards in good view. Inform relevant site staff, particularly if the water supply is to be disrupted, and ensure all staff fully understand the length of service loss and relevant consequences.

Ensure that all outlets are clearly labeled with laminated warning signs quoting “CHEMICAL DISINFECTION IN PROGRESS DO NOT USE WATER”.

This will ensure the complete safety of all people in the area during the work.

Before commencing work flush the incoming water at full bore for at least 10mins, this must be done before every chlorination starts with no exceptions. Whilst flushing measure flow rate in L/min and measure pH and free chlorine levels and record on the job sheet.

Break into incoming pipework/or locate pipe end and attach relevant adaptors to allow fitting of a clean hose.

All hoses used must be clean, previously disinfected, and free from damage. Ensure double-check valves are in place to prevent backflow.

If swabbing is required flush the pipe work with 2 volumes of water at maximum pressure and/or pass chlorinated foam swab into the system and push down the pipe work with water. Remove chlorinated foam swabs.

Attach hoses from incoming mains/water supply to the inlet side of chlorination equipment. Test flow is present through the system.

Attach the hose from the outlet side of chlorination equipment to the feed side of the main feed system.

Place pickup pipe of chlorination equipment into calcium hypochlorite solution in bunded container and prime.

Ensure connections are tight and open the main stop valve.

Open outlets and pull chlorinated water through to all outlets, ensuring >50ppm to 75ppm of chorine appears at all outlets.

Measure free chlorine content at pipe end or nearest and furthest outlets on each leg of pipework with a digital photometer to gain an accurate reading of free chlorine level and note results.

Once adequate free chlorine levels have been achieved at all outlets turn off all valves to further protect against backflow.

Allow the chlorine solution to stand within the pipe work for at least 1 hour or overnight.

Turn back on all valves and turn off chlorination equipment.

Open main stop tap/water supply tap.

Check chlorine levels at outlets to ensure they have remained >35 ppm and record results. If levels have dropped below 35 ppm then return to section 8 and re-start the chlorination procedure.

If chlorine levels are >35 ppm then sample source water for microbiology, label, and place in a cool bag.

Begin to draw water through the system to remove chlorinated water. If chlorinated water is not entering a specific foul drain or is of high volume (>500 L) then de-chlorination of the water must be undertaken.

Sodium metabisulphite or sodium thiosulphate should be dosed into the outgoing water to fully neutralize the chlorine. Regular checks should be made to ensure all chlorine has been neutralized.

Continue to flush outlets until all outlets are showing identical chlorine levels to that of the incoming mains water.

Take the required number of water samples and sample water for free chlorine, total chlorine, and turbidity as standard.

If required perform further chemistry samples and note all results.

Take microbiological water samples from the furthest outlet and label them as “Sample 2, name of job and outlet” and take any further samples required by the client. Check water for odor, taste, and appearance and note results.

Shut off the main stop tap/water supply and disconnect chlorination equipment. Take care to minimize the loss of water from the system.

Reconnect main system pipe work originally broken and turn on mains stop tap to check for leaks or cap off pipe ends with disinfected end caps.

Remove lock-offs and signs, sign off certificate, inform site staff and sign off any work permits. Remove all debris and tools from the site prior to signing out.


All Required training for Stipulated installation activities is provided to all contractor’s technical staff for ensuring quality workmanship.

Working Hours

Site operational time shall be followed as per project progress requirements, and all relevant local authority schedules of holidays are to be observed.


Working Areas will be kept clean by disposing of the waste materials in the designated areas. At the end of the day’s work, the installation supervisor will ensure that the debris is disposed of on regular basis.


Subject to main contractors programmer and scope/area available for MEP activities, but not limited to below:
MEP Supervisor
Plumbers, pipefitters
Assistant fitters


Ensure the material being used are of the approved manufacturer from the project vendor list and of quality as specified.

Plant and Equipment

Flushing & Cleaning Agents
Water Storage Tanks
Portable Hand Tools- Grinder/Hammer-drill
Brush and cleaning waste/cloth.
Spirit Level & Water level pipe
Standard Tool Box Set With Tools
Flood Lights (For Night Works)
Measurement Tape

QC Approval

Follow the approved Method Statement, ITP, QCP, HSE Plan, and Material Approval & Checklist.

Supporting Documents

This Method Statement for Flushing & Disinfection of Potable Water Network is to be read in conjunction with the below-referenced documents
Contract Specification & Approved drawings.
Approved Value Engineering
Project Quality Plan
Project HSE Plan


Copies of this Method Statement for Flushing & Disinfection of Potable Water Network will be circulated to the following for information & action.

Sr. Construction Manager
Site Engineer
QA/QC Engineer
HSE Engineer


Risk Assessment
Inspection & Test Plan
Quality Control Checklists

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