/ Water Safety Plan Guide
Filtration –
Diatomaceous Earth Filtration
Version 1, Ref P6.4
January 2014

Citation: Ministry of Health. 2014. Water Safety Plan Guide: Filtration – Diatomaceous Earth Filtration, Version 1, ref p6.4. Wellington: Ministry of Health.

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

ISBN: 978-0-478-42736-3 (print)
ISBN: 978-0-478-42737-0 (online)

Previously published in 2002 as Public Health Risk Management Plan Guide: Filtration – Diatomaceous Earth Filtration, Version 1, ref p6.4. 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.

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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 P6.4Water Safety Plan Guide:1

Version 1, January 2014Filtration – Diatomaceous Earth Filtration

Ref P6.4Water Safety Plan Guide:1

Version 1, January 2014Filtration – Diatomaceous Earth Filtration

Introduction

This Guide is concerned with the use of diatomaceous earth filtration to remove from water:

  • large germs, such as Giardia and Cryptosporidium
  • other particles, some of which may have germs attached to them.

If an event occurs during the operation of the diatomaceous earth filter (ie, it doesn’t work properly), so that particles are not removed, sickness may result from germs, particularly the larger ones, not being removed.

Diatomaceous earth filter operation can present risks to the health and safety of the operators. These are acknowledged, but are not discussed further as such risks are the subject of health and safety in employment legislation.

The improvement in water quality that can be achieved by a diatomaceous earth filter will depend on the raw water quality (see Guide S1.1) and any earlier treatment processes (see appropriate Guides in the P4, P5 and P6 series, and possibly P1.1 and P8.2).

Risk Summary

The event creating the greatest risk involved in diatomaceous earth filtration is poor removal of particles (see P6.4.1).

The most important preventive measures are:

  • to make sure the pre-coat is properly applied (P6.4.1.2)
  • to make sure the diatomaceous earth particle size is correct (see P6.4.1.2).

(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: DWSNZ – Drinking-Water Standards for New Zealand.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: PARTICLES NOT REMOVED
Possible hazards: Germs not removed (particularly Giardia and Cryptosporidium).
Level of risk: High1
P6.4.1.1
Ruptured septum. /
  • Routine inspection and maintenance schedule.
/
  • Soundness of the septum (visual inspection).
  • Pressure drop across the filter.
/
  • Elevated turbidity and particle count in the filtered water.
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Pressure drop lower than usual operational limit.
/
  • Replace the septum.

P6.4.1.2
Ineffective precoat. /
  • Determine diatomaceous earth particle size that will achieve target particle size removal.
  • Ensure pre-coat recirculation cycle is adequate.
  • Determine whether pre-treatment of septum with polyelectrolyte is necessary.
  • Alarm system to warn if diatomaceous earth supply is close to running out.
  • Maintain and make use of records of diatomaceous earth use to provide a guide to the length of time each supply is likely to last.
  • Maintain a reserve supply, adequate to cover the time to deliver an urgent supply.
/
  • Particle size and turbidity in filtered water.
  • Time for pre-coat to be circulated through the filter (ie when the filter is being prepared to be brought back on-line).
  • Diatomaceous earth supply.
  • Diatomaceous earth usage.
/
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Elevated turbidity and particle count in the filtered water.
  • Diatomaceous earth supply exhausted.
/
  • Obtain diatomaceous earth with a more suitable particle size.
  • Increase precoat circulation time.
  • Introduce pre-treatment.
  • Install (or repair) an alarm to warn that the diatomaceous earth is close to running out.
  • Urgently order diatomaceous earth.

1The consequences of the event, and therefore the level of risk, will be influenced by the quality of the source water and the type and effectiveness of any disinfection after diatomaceous earth filtration.

Causes / Preventive measures / Checking preventive measures / Corrective action
What to check / Signs that action is needed
Event: PARTICLES NOT REMOVED cont’d
P6.4.1.3
Ineffective body feed. /
  • Ensure the rate of body feed of diatomaceous earth matches the turbidity of the source water.
  • Diatomaceous earth supply tank fitted with an alarm to indicate when it is close to running out.
  • Maintain and use records of diatomaceous earth use to provide a guide to the length of time each supply is likely to last.
  • Maintain a reserve supply, adequate to cover the time to deliver a replacement supply.
/
  • Particle counts and turbidity in water leaving the filter.
  • Pressure drop across the filter.
  • Diatomaceous earth supply.
  • Diatomaceous earth usage.
/
  • Elevated turbidity and particle count in filtered water.
  • Increased pressure drop across the filter (coat clogs with particulates).
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Diatomaceous earth supply exhausted.
/
  • Adjust feed rate of diatomaceous earth.
  • Install (or repair) an alarm to warn that the diatomaceous earth is close to running out.
  • Urgently order diatomaceous earth.

P6.4.1.4
Loss of coat. /
  • Prevent coat dropping or cracking by operating continuously and avoiding pressure drops.
  • Ensure appropriate procedure for restart should coat drop. Use filtered water to apply the pre-coat.
/
  • Particle counts and turbidity in water leaving the filter.
  • Pressure drop across the filter.
  • Operational down-time (leads to pressure drops).
/
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Elevated turbidity and particle count in filtered water.
  • Abnormally low pressure drop across the filter.
  • Long periods of down-time which may result in loss of coat.
/
  • Drop coat and begin cycle from pre-coat.
  • Review operating procedures to prevent pressure drop.
  • Ensure continuous operation.
  • Review operations manual.

P6.4.1.5
Ineffective backwashing. /
  • Ensure sufficient time is allowed for the backwash so that residual particle matter does not foul the septum during subsequent pre-coats.
/
  • Backwash time.
  • Pressure drop across the filter.
  • Particle counts and turbidity in water leaving the filter.
/
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Elevated turbidity and particle count in filtered water.
/
  • Increase backwash time.

P6.4.1.6
Turbidity too high entering the filter /
  • Provide pre-treatment of the water entering the filter.
/
  • Input water turbidity.
  • Particle counts and turbidity in water leaving the filter.
/
  • Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
  • Elevated turbidity and particle count in filtered water.
/
  • Introduce pre-treatment.
  • Reconsider the suitability of diatomaceous earth filtration for this water source.

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.

Event – Particles and large germs not removed
Indicators: /
  • High turbidity of filtered water.
  • More than 0.01% of 2–15 m-sized particles are detected in filtered water (see Section 3.2.3.1 DWSNZ:2000).
  • Cases of giardiasis and cryptosporidiosis, associated with periods of poor water quality, reported in the community.

Required actions: /
  • Immediately take the treated water off-line. Provide another source of potable water until water of acceptable quality can again be supplied.
  • Identify the cause of inadequate filtration and rectify (see Section 3.4.2.1, DWSNZ:2000).
  • Monitor particle counts until they reach acceptable levels.
  • If inadequately filtered water has entered the reticulation, inform the MOH of the transgression, and assess whether a boil water notice is required.
  • Restart plant operation.
  • Record the cause of the failure and the corrective steps taken.
  • Modify water safety plan if necessary.

Responsibility: / Manager designated responsible for water supplies.

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.

What to measure or observe: /
  • Particle size.
  • Microscopic particle analysis.
  • Follow the protocols set out in Section 3 of the DWSNZ:2000.

How often: /
  • For populations of more than 10,000, periodic filter performance testing and on-line particle counting, as in Table 3.3 of DWSNZ:2000.
  • For populations of 10,000 or fewer, as for populations of more than 10,000, or by direct microscopy as in Table 3.3 of DWSNZ:2000.

What to do with the results: /
  • Record results 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 diatomaceous earth filter.
  • 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 diatomaceous earth filter are recognised in the plan.

Responsibility: /
  • Manager designated responsible for the water supply.

Ref P6.4Water Safety Plan Guide:1

Version 1, January 2014Filtration – Diatomaceous Earth Filtration