Water Safety Plan Guide: Distribution System – Post ... · Web viewDistribution System – Post-Treatment Storage Version 1, January 2014 Water Safety Plan Guide Distribution System

Water Safety Plan Guide

Distribution System – Post-treatment Storage

Version 1, Ref D1

January 2014

Citation: Ministry of Health. 2014. Water Safety Plan Guide: Distribution System – Post-treatment Storage, Version 1, ref

D1. Wellington: Ministry of Health.

Published in January 2014 by the Ministry of Health

PO Box 5013, Wellington, New Zealand

ISBN: 978-0-478-42762-2 (print)ISBN: 978-0-478-42763-9 (online)

Previously published in 2001 as Public Health Risk Management Plan Guide: Distribution System – Post-

treatment Storage, Version 1, ref D1. This publication’s title and any reference within the text to ‘public health risk

management plan’ were changed in January 2014 to reflect the December 2013 legislation change of the term ‘public health risk management plan’ to ‘water safety plan’. No

other changes have been made to this document.

This document is available at: www.health.govt.nz

This work is licensed under the Creative Commons Attribution 4.0 International licence. In essence, you are free to: share ie, copy and redistribute the material in any medium or format; adapt ie, remix, transform and build upon the material. You must give appropriate credit, provide a link to the licence and indicate if changes were made.

Contents

IntroductionRisk SummaryRisk Information TableContingency PlansWater Safety Plan Performance Assessment

iv [Title]

Ref D1 Water Safety Plan Guide: vVersion 1, January 2014Distribution System – Post-Treatment Storage

IntroductionPost-treatment storage facilities (service reservoirs) are used in many water supplies to: equalise demands on:

– water sources– treatment plant– pumping stations– distribution mains

maintain uniform pressures in the distribution system provide reserves for emergencies such as:

– power failure– fires– equipment failures in the supply.

This Guide is concerned with tanks and reservoirs used for this purpose. These facilities should not alter the quality of the water.

If an event occurs during the storage of treated water (ie, something happens to affect the quality or quantity of water available from the storage facility), the following could happen: If there is not enough water in storage to meet demand, sickness

may be caused by poor hygiene and by low pressure allowing germs and chemicals to enter the distribution system.

If contamination gets into the tank or reservoir, germs or chemicals can cause sickness.

If sediment settles on the bottom of the tank or reservoir, or is stirred up, germs or chemicals in the sediment can cause sickness.

Maintenance of reservoirs can present risks to the health of staff. These are acknowledged, but are not discussed further as such risks are the subject of health and safety in employment legislation. Shortage of water can lead to fire-fighting problems, and hence to the possibility of injury. This is not dealt with in this Guide.

Post-treatment storage and the risks associated with it cannot be viewed in isolation. Having enough storage in the reservoirs depends on: being able to abstract enough water of adequate quality from the

source (see P1 series of Guides) the operation of the treatment plant at a rate at which it can treat

the water (see Guide P11) the system pumps working (see Guide P10)

Ref D1 Water Safety Plan Guide: 1Version 1, January 2014Distribution System – Post-Treatment Storage

being able to transfer water from the treatment plant to the storage facility (see Guide P2)

the community water demand.

Having water stored of acceptable quality depends on: the quality of the source water (see the S1 and P1 series of Guides) the good operation of treatment processes before storage (see the

P4, P5, P6, P7 and P8 series of Guides) events that affect the quality of the water once it is in the reservoir:

– introduction of contaminated material into service reservoirs: access by animals/birds/humans build up of sediment or living organisms (including algal

growth) (see Guides S1.1 and P4.1) construction materials dissolving or corroding contaminated water running in from the surroundings or

from the roof– short-circuiting; disinfection is incomplete because the water

does not stay in the reservoir long enough (see P7 series of Guides)

– regrowth/resuspension of material within service reservoir: biological regrowth or the disturbance of sediment or

living organisms post-treatment floc formation.

2 Water Safety Plan Guide: Ref D1Distribution System – Post-Treatment Storage Version 1, January

2014

Risk SummaryThe events creating the two greatest risks involved in treated water storage are having too little water stored to meet demand (see D1.1), and contamination getting into the reservoir (see D1.2).

The most important preventive measures are: construct another reservoir or tank if having too little water seems

like being a continuing problem (see D1.1.3) restrict access to the reservoir to reduce the chances of

contamination (see D1.2.1–3) carry out regular inspections for signs of structural deterioration or

of contamination having got into the storage (see D1.2.1) make sure the facilities are designed to reduce the chances of

contamination getting in, and that the construction materials will not contaminate the water (see D1.2.3–6)

make sure there is enough disinfectant in the water to kill germs that get into the water (see D1.2.1–3).

(References in parentheses are to the Risk Information Table.)


Risk Information TableReliable information about water quality is essential for the proper management of a water supply. Knowledgeable and skilled staff are also essential for minimising the public health risks associated with water supplies. Please read the staff training (Guide G1) and the monitoring guides (Guide G2). While we haven’t pointed out every detail of how these documents are linked with the present document, the links are many and are important.

Abbreviations: MAV – Maximum acceptable value – see Drinking-Water Standards for New Zealand:2000; NTU – nephelometric turbidity units

Causes Preventive measures Checking preventive measures Corrective action

What to check Signs that action is needed

Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMANDPossible hazards: Low pressure in the water supply system may allow entry of germs and chemical determinands; hazards associated with poor hygiene.Level of risk: High

D1.1.1Insufficient supply of raw water.

Provide sufficient raw water storage to cover seasonal variations in source water availability, periods of untreatable raw water quality, maximum limits on consent for abstraction from the source.

Develop auxiliary sources that can be used when secondary sources cannot meet demand.

Refer also to Guide P1 series.

Pre-treatment storage levels.

Raw water storage levels or source flows unacceptably low.

Plan development of new sources.

Restrict development in distribution area until adequate water available.

D1.1.2Insufficient water treatment capacity, or output limited by one or more treatment stages not operating.

Design treatment plant to meet expected demand for projected life of the plant.

Identify treatment stages that are limiting capacity and undertake modification of these.

Ensure appropriate water safety plans are operational for all treatment processes to avoid or limit down time.

Post-treatment storage levels unacceptably low.

Calculate future water use and plan actions to meet likely demand.

Identify problem treatment stage and increase throughput if possible.


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D1.1.3Inadequate post-treatment storage capacity.

Construct additional storage.

Encourage water conservation during periods when demand is likely to be high.

Water consumption.

Need for water use restrictions.

Post-treatment storage levels unacceptably low.

Water use restrictions.

Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMAND cont’d

D1.1.4Inability to transmit water from source to plant, or plant to post-treatment storage (including power failure).

Ensure pumps have stand-by generators.

Routine inspection and maintenance of pipelines.

Inspection log.

Electricity supply.

Water transmission ceases.

Maintenance and inspection not carried out.

Re-fuel generators (if necessary).

Clear blockages/ repair pump breakdowns or pipe failures.

D1.1.5Leakage from the storage facility.

Carry out routine testing of storage for leaks.

Change in storage level.

Change in storage level does not equal water input minus water output (including evaporation).

Locate and fix leak.

Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIRPossible hazards: Germs, arsenic, copper, chromium, iron, chlorine, aluminium.Level of risk: High1




D1.2.1Access by animals/birds.

For tanks in general:– ensure the storage is

covered– install mesh, grills,

vents, etc– undertake regular

tank inspections– document condition

of structure, mesh, grills, vents, tank liner (for gaps) etc and take action.

For wooden tanks:– install metal band

around wooden tanks to prevent access from ground level

– remove vegetation near tanks

– install internal liner for tank roof.

Maintain an FAC residual.

Inspection reports.

Free available chlorine (FAC).

Unexplained change in water quality.

Taste and odour complaints.

Visual evidence of entry of animal or bird.

FAC concentration less than 0.2 mg/L.

Replace damaged screens, etc.

Isolate tank, chlorinate and check the microbiological quality. If there is gross contamination, dump the water, clean and refill before chlorinating.

1 The level of risk from microbiological contamination is less if a chlorine residual is maintained.



Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d

D1.2.2Unauthorised access/ vandalism/ sabotage.

Provide:– locked perimeter

fence– locked access towers

and ladders– locked hatches on

tanks– intruder alarm.

Maintain chlorine residual.

Regular site inspections with checks on security measures.

FAC. Microbiologica

l quality.

Evidence of break-ins or damage.


FAC residual less than 0.2 mg/L and cannot be maintained.

E. coli or coliforms detected in 100 mL of water.

See Contingency Plan.

Determine how access was obtained and rectify shortcoming in security.



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D1.2.3Human access for sampling, etc.

Train personnel who need access to tanks.

Design internal walkways that do not become submerged with a solid floor and raised edges to avoid shoe scrapings entering the water.


Isolation of tank. Procedures for staff

entering tanks to clean footwear, then to cover their shoes with clean plastic bags, or to go through a chlorinated foot-bath.

Test sampling staff to ensure they are not carriers of water-borne disease, and relieve them of sampling duties if necessary.

Microbiological quality.

Reservoir construction.

Deterioration of water quality following operation involving tank access, in particular E. coli or coliforms detected in 100 mL of water.


D1.2.4Leaching/ corrosion from construction materials.

Specify materials certified for potable water use2 in tenders and check these specifications are being met during construction.

Use accredited contractors.

Low residence time of water in tanks (while maintaining disinfection contact time).

Conduct regular inspection of tanks.

Check residence times.

If roof is unlined – copper/ chromium/ arsenic (especially after heavy rain).

Deterioration of water quality following new installation.

Change in water quality after rain.

Concentration of copper, chromium or arsenic is more than 50% of its MAV.

Replace sub-standard materials.

Install roof liners in tanks with treated timber roofs.

Replace lid with more suitable material.

2 See Guide D2.1 regarding approved materials.





D1.2.5Entry of roof drainage.

Specify and audit roof design:– rainwater to drain

from (not into) tank– restrict access to

roof top– hatches designed to

exclude water ingress.

FAC. Microbiologica

l quality. Installation

design. Annual roof

inspection report.

Change in water quality following rain.


Flaws in the design of the roof.

Repair leaky joints/cracks/ hatches promptly.

Replace badly designed hatches.

D1.2.6Entry of contaminated groundwater (for in-ground tanks).

Repair leaks promptly. Inspect tank every 1–5

years (depending on the age of the tank and risks caused by its location).


Tank inspection report.



Repair leaks. Isolate tank,

chlorinate and check the microbiological quality. If there is gross contamination, dump the water, clean and refill before chlorinating.

Line the tank with a suitable waterproof material.

Leaks identified by the inspection.

Complaints of petrol smell (if near underground fuel tanks).

D1.2.7Animal entry from upstream source (animals trapped and destroyed by action of control valves) – where the tank is filled directly from the source.

Ensure there are screens on the source outlet.

Regular screen inspection.


Turbidity.

Taste and odour complaints.


Turbidity more than 0.2 NTU.

Replace and clean screens.



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D1.2.8Chemical contamination from incorrect chemical dosing.

Train staff in calculation of dose rates and in the use of methods for optimising treatment plant operation.

Document treatment plant dosing procedures (especially calculations of type and quantity of materials used), and those for the reservoir if it is dosed.

Treatment chemicals.

Turbidity.

Elevated turbidity/ treatment chemical concentration post- treatment.

Absence of written descriptions of dosing procedures.


See Contingency Plan.


D1.2.9Backflow from drains.

Ensure that there is an airgap or other suitable backflow prevention device between tank overflow and sewer or stormwater drains.


Turbidity. Overflow

construction.

Change in water quality after rain events.



Disconnect any direct connections.

Isolate tank, dump the water, clean, chlorinate and check the microbiological quality before refilling.

Event: DEVELOPMENT, OR RE-SUSPENSION, OF SEDIMENT WITHIN TANK OR RESERVOIRPossible hazards: Germs; manganese, disinfection by-products (trihalomethanes, haloacetic acids, chloral hydrate).Level of risk: Low–moderate




D1.3.1Sediment/ slime accumulation and release.

Avoidance of rapid drain/fill.

Undertake regular cleaning of reservoirs (every 1–5 years depending on the quality of the source water, flow through and degree of stagnation in the tank etc).

Cleaning procedures need to consider disinfection of all equipment, satisfactory training procedures, preferably isolation during cleaning, minimisation of sediment stir-up if online during cleaning.


Minimise conditions leading to post-treatment floc formation.

FAC. Microbiologica

l quality. Turbidity. Manganese. Audit cleaning

procedures and operation.

Visible slime/ sediment.

Customers complain of slime/ sediment.

FAC residual concentration less than 0.2 mg/L.

E. coli or coliforms detected in 100 ml of water.


Review cleaning procedures and plant operation.


Event: CHLORINE CONTACT TIME TOO SHORTPossible hazards: Germs not killed.Level of risk: High

D1.4.1Storage tank too small.

Check calculations of tank hydraulics and the hydraulic profile of the network before construction; make use of hydraulic models if necessary.

Flow rates (measured through tank).


Installation design.


E. coli or coliforms detected in 100 mL of water despite adequate FAC residual concentration.

Unexplained fluctuations in water quality.

Increase size of tank.

Event: CHLORINE CONTACT TIME TOO SHORT (cont’d)

D1.4.2Short-circuiting.

Specify and audit suitable tank design to include:– baffles– fill at the top and

empty from the bottom.

If necessary, redesign the inlet/outlet or install baffles.

Flow rates (measured through tank).


Installation design.


E. coli or coliforms detected in 100 mL of water despite adequate FAC residual concentration.

Unexplained fluctuations in water quality.

Operate the tank to force turnover (deliberately run up and down).


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Contingency PlansIf an event happens despite preventive and corrective actions you have taken, you may need to consult with the Medical Officer of Health to assess how serious a problem is.

Event – Breach of the post-treatment storage tank/reservoir

Indicators: Visual inspection shows evidence of structural deterioration or a leak.

Water levels cannot be maintained despite water being pumped into storage at an adequate rate.

Required actions:

Identify the location of the leak, and repair. Bypass the tank/reservoir and take water directly

from the treatment plant. If the normal water source does not meet the

needs of the community or treatment plant capacity (ie, the quantity is too low at that time of year):– investigate and obtain alternative source(s) of

water– implement water use restrictions.

Repair the storage tank/reservoir if a leak is the cause of the shortage.

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.

Responsibility:

Manager designated responsible for the water supply.


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Event – Germs detected in the stored water

Indicators: A detectable chlorine residual cannot be obtained in the water leaving the reservoir.

In 100 ml samples of water from the reservoir, E. coli or coliforms are continually detectable, or E. coli is present at elevated levels (more than 10 per 100 mL).

Widespread levels of illness in the community. Knowledge of sabotage or vandalism.

Required actions:

Follow the actions given in Figure 3.3 of the DWSNZ:2000. In consultation with the MOH, consider dumping the stored water, even though indicators are clear, if there have been high levels of microbiological contamination.



Responsibility:



Event – High levels of chemical contaminants in the stored water

Indicators: Knowledge of a major chemical spill, or chemical overdose (eg, chlorine), into the reservoir; sabotage or vandalism.

Change in the appearance, smell or taste of the water.

Widespread levels of illness in the community. Chlorine residual not restored.

Required actions:

Close down the reservoir, and if necessary the supply. Provide another source of potable water until water of acceptable quality can again be supplied.

Inform the MOH of the situation. Identify the source of contamination and take steps

to avoid recontamination. Dump the reservoir water. Consultation with the

regional council will probably be required with regard to disposal of the flushed water.

Flush the reticulation system and monitor water quality until determinand concentrations are again less than 50% of the MAV.

Warn consumers to thoroughly flush their taps before drawing water for use.



Responsibility:



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Water Safety Plan Performance AssessmentTo make sure that your supply’s water safety plan (formerly known as a Public Health Risk Management Plan, PHRMP) is working properly, periodic checks are needed. The overview document outlines what needs to be done. The following table provides the detailed information for checking this particular supply element.

What to measure or observe:

Inspection and sampling of service reservoirs. E. coli and coliforms Turbidity Chemical Priority 2 determinands (if any exist) FAC.Follow the protocols set out in DWSNZ:2000.

How often: For the monitoring frequencies for E. coli, turbidity and FAC measurements see DWSNZ:2000 Section 3.3.2. For chemical determinands see DWSNZ:2000 Table 4.1.

Other regular inspection and sampling should include, but not be limited to, the following:– evidence of human or animal access– assessment of security– evidence of cracks or holes that may allow

contaminants to enter– appearance and odour of the water.

The frequency will be site-dependent and should be based on previous findings.


What to do with the results:

Results need to be recorded to meet legislative requirements or to allow water safety plan performance assessment. The WINZ database is good for this.

The collected data need to be periodically reviewed to see whether problems with this supply element are developing. This should be done as frequently as the manager responsible considers necessary to minimise risk to public health arising from this supply element.

Should this review show any unusual incidents, indicate that proper procedures are not being carried out, highlight poor laboratory results or indicate that poor water quality is reaching customers, then review the procedures for managing service reservoirs.

Evaluate the monitoring results, and any actions taken as the result of having to implement a contingency plan, to see if the water safety plan needs modification – eg, preventive measures are up to date; the contingency plan steps are still adequate; and changes to the operation of post-treatment storage are recognised in the plan.

Responsibility: Manager designated responsible for the water supply.


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Documents

Water Safety Plan Guide: Distribution System – Post ... · Web viewDistribution System – Post-Treatment Storage Version 1, January 2014 Water Safety Plan Guide Distribution System