Water Safety Plan Guide: Pre-Treatment Processes Destratification

Citation: Ministry of Health. 2014. Water Safety Plan Guide: Pre-treatment Processes – Destratification, Version 1, ref p4.2. Wellington: Ministry of Health.

Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand

ISBN: 978-0-478-42718-9 (print)
ISBN: 978-0-478-42719-6 (online)

Previously published in 2001 as Public Health Risk Management Plan Guide: Pre-treatment Processes – Destratification, Version 1, ref p4.2. This publication’s title and any reference within the text to ‘public health risk management plan’ was 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

Introduction

Risk Summary

Risk Information Table

Contingency Plans

Water Safety Plan Performance Assessment

Ref P4.2Water Safety Plan Guide: 1

Version 1, January 2014Pre-Treatment Processes – Destratification

Ref P4.2Water Safety Plan Guide: 1

Version 1, January 2014Pre-Treatment Processes – Destratification

Introduction

Destratification is used in lakes and reservoirs to avoid the formation of layers of water that have little or no oxygen. This Guide is concerned with destratification by both aeration and hydraulic mixing.

If an event occurs during the destratification process (ie, the water is not properly mixed), the following could happen:

• large growths of algae develop
• the raw water becomes unsuitable for treatment
• high concentrations of manganese and iron develop
• variability in water quality makes treatment difficult to control.

These can lead sickness caused by germs, disinfection by-products, manganese or algal toxins.

The operation of the equipment for destratification 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.

The destratification process, and the risks associated with it, cannot be viewed in isolation. They influence elements of the water supply that are dealt with in other Guides. The following elements can be affected:

• chemical disinfection (see Guides P7.1, 7.2, 7.3)

– water that is low in oxygen often contains substances that react quickly with disinfectants so that not enough disinfectant is left to kill germs and disinfectant by-products exceed the MAV

– oxidation of iron and manganese can increase the turbidity of the water which hinders the disinfectants in killing germs

• ultraviolet disinfection (see Guide P7.4)

– iron and manganese may foul the lamps or increase the turbidity of the water which stops the ultraviolet light getting to the germs

• processes removing natural organic matter (eg, coagulation/flocculation – see Guides P5.1, 5.2, 5.3)
• processes that are pH sensitive (eg, coagulation/flocculation).

Risk Summary

The event creating the greatest risk involved in destratification is poor mixing of the water (see P4.2.1).

The most important preventive measures are:

• good aerator design (see P4.2.1.1)
• having intakes at different levels in the lake/reservoir or having a single intake that can be used at a range of depths (P4.2.1.3)
• starting the destratification process well before the different layers in the water are likely to start forming (P4.2.1.4).

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

Risk Information Table

Reliable 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; DWSNZ – Drinking-Water Standards for New Zealand

• Use a design that has been successfully used previously.
• In a long narrow reservoir (eg, a dammed river) use a longitudinal aerator. It will be more hydraulically efficient than a point source aerator.

• Dissolved oxygen (DO) with depth.
• Temperature with depth, to identify the formation of a thermocline.3
• FAC (free available chlorine) leaving the treatment plant and through the distribution system.
• Iron.
• Manganese.
• Ammonia.

• Large differences between DO and temperature levels at different depths.
• Elevated levels of Fe and Mn entering the intake.
• Complaints of tastes and odours.
• FAC concentration below 0.2 mg/L.

• Redesign destratification unit.
• Redirect intake to upper levels of the water body until destratification can be resumed.
• Use low-level intake to aid destratification in the springtime and early summer.

• Undertake studies to determine the location that will achieve the best mixing.

• Determine best location and move destratification unit.

1Disinfectant demand is the difference between the amount of disinfectant added to the water and the disinfectant residual concentration remaining after the disinfectant has reacted with other substances in the water.

2The consequences of the event will depend on the chemical determinands that appear in the water, and the efficacy of subsequent treatment processes in removing them from the water. The risk will be high if algal toxins are in the water.

3Thermocline is a region of rapid change in water temperature with the depth.

• Design the intake system with multiple abstraction levels or with a variable level of abstraction.

• Intake structure has single fixed level of abstraction.
• Large differences between DO and temperature levels at different depths.
• Elevated levels of iron and manganese entering the intake.
• Complaints of tastes and odours.
• FAC concentration below 0.2 mg/L.

• Redesign intake structure.
• Redirect intake to avoid anaerobic water.

• Monitor water quality and physical parameters on an annual basis to assess when stratification is likely to become a problem. Start destratification before then.

• Start data collection so that the problem can be avoided next year.
• Redirect intake to avoid anaerobic water.

• Routine preventive maintenance schedule.
• Replace suspect units.

• Maintenance schedule (see also P4.2.1.1).

• Maintenance log shows frequent maintenance needed (see also P4.2.1.1).

• Start preventive maintenance programme.

• Stand-by electricity generator.
• Reserve fuel for generators.

• Electricity supply (see also P4.2.1.1).

• Poor continuity of power supply (see also P4.2.1.1).

• Refuel generator (if appropriate).
• Obtain more fuel.

Contingency Plans

If 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.

• Dead fish in the water body.
• Human sickness consistent with algal toxin poisoning.
• Visual evidence of algal mats developing in the lake or reservoir.

• Notify the MOH, close down the supply. Provide another source of potable water until water of acceptable quality can again be supplied.
• In conjunction with the MOH, evaluate the risk to health posed by algal toxins that have been formed, or are likely to form. This will require knowledge of the algae present, the nature of toxins released, and their susceptibility to oxidation.
• Consider (see Guide S1.1):

• Once corrective measures are in place, monitor algal levels in the raw water and assess whether toxin concentrations in the treated water are acceptable before again reticulating the water.
• Flush the distribution system.
• Warn consumers to flush their taps before resuming the supply of water.
• Record cause of system failure and steps taken to correct.
• Modify water safety plan if necessary.

• A detectable chlorine residual cannot be obtained in the water leaving the treatment plant (high chlorine demand due to poor destratification).
• In 100 mL samples of water leaving the treatment plant, 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.

• Follow the actions given in Figure 3.2 of the DWSNZ:2000.
• Identify the reason for the failure and rectify.
• Record cause of system failure and steps taken to correct.
• Modify water safety plan if necessary.

Water Safety Plan Performance Assessment

To 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.

• Dissolved oxygen (DO) and temperature levels (at different depths) in the water body
• Iron, manganese, ammonia and total organic carbon (TOC) levels entering the plant intake
• FAC levels leaving the treatment plant
• The operation of the motors driving the aerators or pumps.

• FAC should be monitored as required by the DWSNZ:2000, as should manganese if a P2 determinand
• The determinands noted above could be monitored at a frequency that the water supplier considers necessary for process control. The easily-made measures of temperature and DO could be made daily, although once results show that the destratification unit is operation successfully it may be sufficient to ensure that the motors are operating.

• 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 the treatment plant.
• 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 destratification process are recognised in the plan.

Ref P4.2Water Safety Plan Guide: 1

Version 1, January 2014Pre-Treatment Processes – Destratification