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.
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
1
Risk Summary
2
Risk Information Table
3
Contingency Plans
5
Water Safety Plan Performance Assessment
6
Ref P6.4
Version 1, January 2014
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
iii
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).
Ref P6.4
Version 1, January 2014
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
1
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.)
2
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
Ref P6.4
Version 1, January 2014
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
Corrective
action
Checking preventive measures
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.
P6.4.1.2

Ineffective
pre-coat.
Determine
diatomaceous earth
particle size that will
achieve target particle
size removal.

Ensure pre-coat
recirculation cycle is
adequate.

Determine whether pretreatment of septum
with polyelectrolyte is
necessary.



1
Routine inspection and
maintenance schedule.
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.

Soundness of
the septum
(visual
inspection).


Pressure drop

across the filter.
Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.

Pressure drop
lower than usual
operational limit.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.

Particle size
and turbidity in
filtered water.

Time for precoat 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.
Elevated turbidity 
and particle count
in the filtered
water.

Elevated turbidity
and particle count 
in the filtered
water.
Diatomaceous
earth supply
exhausted.
Replace the
septum.
Obtain
diatomaceous
earth with a
more suitable
particle size.
Increase
pre-coat
circulation
time.

Introduce pretreatment.

Install (or
repair) an
alarm to warn
that the
diatomaceous
earth is close
to running
out.

Urgently order
diatomaceous
earth.
Maintain a reserve
supply, adequate to
cover the time to deliver
an urgent supply.
The 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.
Ref P6.4
Version 1, January 2014
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
3
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: PARTICLES NOT REMOVED cont’d

P6.4.1.3
Ineffective
body feed.


P6.4.1.4
4

Pressure drop
across the filter.

Diatomaceous
earth supply.

Diatomaceous
earth usage.
Maintain and use
records of
diatomaceous earth use
to provide a guide to the
length of time each
supply is likely to last.

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.

Ineffective
backwashing.

Turbidity too
high entering
the filter
Particle counts
and turbidity in
water leaving
the filter.
Maintain a reserve
supply, adequate to
cover the time to deliver
a replacement supply.

P6.4.1.6
Diatomaceous earth
supply tank fitted with
an alarm to indicate
when it is close to
running out.


Loss of coat.
P6.4.1.5
Ensure the rate of body
feed of diatomaceous
earth matches the
turbidity of the source
water.
Particle counts
and turbidity in
water leaving
the filter.

Elevated turbidity 
and particle count
in filtered water.

Increased
pressure drop
across the filter
(coat clogs with
particulates).


Diatomaceous
earth supply
exhausted.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Pressure drop

across the filter.
Operational
down-time
(leads to
pressure
drops).
Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.


Long periods of
down-time which
may result in loss
of coat.
Ensure sufficient time is 
allowed for the

backwash so that
residual particle matter

does not foul the
septum during
subsequent pre-coats.
Backwash time. 
Particle counts
and turbidity in
water leaving
the filter.

Elevated turbidity
and particle count
in filtered water.
Provide pre-treatment of 
the water entering the
filter.

Input water
turbidity.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Pressure drop
across the filter.
Particle counts
and turbidity in
water leaving
the filter.
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration


Install (or
repair) an alarm
to warn that the
diatomaceous
earth is close to
running out.

Urgently order
diatomaceous
earth.

Drop coat and
begin cycle
from pre-coat.

Review
operating
procedures to
prevent
pressure drop.

Ensure
continuous
operation.

Review
operations
manual.

Increase
backwash time.

Introduce pretreatment.

Reconsider the
suitability of
diatomaceous
earth filtration
for this water
source.
Elevated turbidity
and particle count
in filtered water.
Abnormally low
pressure drop
across the filter.
Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Elevated turbidity
and particle count
in filtered water.
Adjust feed rate
of
diatomaceous
earth.
Ref P6.4
Version 1, January 2014
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:
Required actions:
Responsibility:
Ref P6.4
Version 1, January 2014

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.

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.
Manager designated responsible for water supplies.
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
5
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:
How often:
What to do with the
results:
Responsibility:
6

Particle size.

Microscopic particle analysis.

Follow the protocols set out in Section 3 of the
DWSNZ:2000.

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.

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.

Manager designated responsible for the water supply.
Water Safety Plan Guide:
Filtration – Diatomaceous Earth Filtration
Ref P6.4
Version 1, January 2014