Water Safety Plan
Guide
Treatment Processes
– Rapid Sand Filtration
Version 1, Ref P6.1
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration, Version 1, ref p6.1.
Wellington: Ministry of Health
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42730-1 (print)
ISBN: 978-0-478-42731-8 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Rapid Sand Filtration, Version 1,
ref p6.1. 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 on the Ministry of Health’s website:
http://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
3
Risk Information Table
4
Contingency Plans
7
Water Safety Plan Performance Assessment
8
Ref P6.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
iii
Introduction
Sand filtration is used to remove particles from drinking-water. These particles include:

silts and clays

flocs from the coagulation/flocculation process, or insoluble substances formed from
other treatment processes

larger germs such as Giardia and Cryptosporidium

particles of organic matter, or with organic matter attached to them.
This Guide is concerned with all sand filter types except slow sand filters. (For information
on slow sand filters, see Guide P6.2.)
If an event occurs during the filtration process (ie, it does not work properly), the following
could happen:

If particles are not removed, sickness can come from larger germs not being removed

If natural organic matter is not removed, it reacts with the disinfectant and can cause
sickness from germs not being killed, and from disinfection by-products

If chemicals used in the coagulation/flocculation process are carried into the
distribution system, they could cause sickness.
The operation of filters can present risks to the health of treatment plant staff. These are
acknowledged, but are not discussed further as such risks are the subject of health and safety
in employment legislation.
Sand filters can be used in any of three ways:

In direct filtration, in which coagulant or other chemicals are dosed before the filter:
–
coagulant addition with low turbidity raw water (see Guide P5.3)
–
oxidant addition to remove iron and manganese (see Guide P8.2)
–
activated carbon addition to remove very low concentrations of organic
substances (see Guide P8.4)

As a polishing filter, after conventional coagulation/flocculation/sedimentation or
dissolved air filtration (see Guides 5.1 and 5.2)

As a roughing filter to remove coarser particles before final filtration using a different
filter type (see other Guides in the P6 series).
Several factors influence how effective a rapid sand filter is:

source water quality; poor quality water or water of variable quality makes it more
difficult for a filter to produce good quality water (see Guide S1.1 and Guides in the
P1 series)

how well treatment steps before the filter are working; if more particles can be
removed before the filter, production of a good quality water is easier (see Guides in
the P1, P3, P4, and P5 series)
Ref P6.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
1

the type and depth of the filter media; removal of particles is improved by the use of
more than one type of filter material and by greater depths of these materials

the use of chemicals to coat the sand grains with substances that improve the
attachment of particles to them

design of the filter so that it matches the raw water quality and the operational
requirements of the treatment plant

the rate at which water passes through the filter; slower rates will give better removal
of particles, sudden increases in rate will shake loose particles that are have already
been trapped in the sand, causing “spikes” in the turbidity

the way the filter is operated; how often the filter is backwashed, how the filter is
restarted, the management of the filter ripening and the procedures used in cleaning
the filter sand, for example.
Filtration is the last treatment stage that can physically remove contaminants before
disinfection. The effectiveness of this stage is therefore very important, because particles in
the water hinder germs being killed by the disinfectant, and because the large germs (eg,
Cryptosporidium) that cannot be killed by chlorine have to be physically removed.
2
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
Ref P6.1
Version 1, January 2014
Risk Summary
The event creating the greatest risk with rapid sand filtration is the poor removal of particles
(see P6.1.1).
The most important preventive measure is to make sure the depth and type of sand and the
flow rate are suitable for the quality of water being filtered (see P6.1.1.1).
(References in parentheses are to the Risk Information Table.)
Ref P6.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
3
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: NTU – nephelometric turbidity units; DWSNZ – Drinking-Water Standards for New Zealand
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: PARTICLES NOT REMOVED
Possible hazards: Germs not removed (particularly Giardia and Cryptosporidium).
Level of risk: High1

P6.1.1.1
Media
deficiencies.

P6.1.1.2
Inappropriate
flow rates.
1
4
Ensure depth and type
of medium are suitable
for the quality of the
water being filtered and
flow rate.
Determine and
document acceptable
flow rates, and check
regularly. Include
summer versus winter
flows and ensure both
can be matched to
media specifications.

Avoid sudden changes
in flow rates in dirty
filters.

Ensure operators are
trained in correct
operational procedures
for the specific filters in
use.

Visual inspection
of filter.

Post-filter:
–
–

Turbidity
Aluminium
residual
concentration.
Post-filter:
–
–

Turbidity more
than 0.5 NTU
after the filter.

Change type,
number and/or
depth of filter
media.

Aluminium
residual more
than 0.15 mg/L
after filter.

Frequent
elevated
turbidity in
filtered water.

Turbidity more
than 0.5 NTU
after the filter.

Restrict
maximum flow
rate.

Aluminium
residual more
than 0.15 mg/L
after filter.

Redesign filter
hydraulics.

Frequent
elevated
turbidity in
filtered water.

Modify
operational
practices.

Identify staff
training needs
and provide
training.
Turbidity
Aluminium
residual
concentration.
The consequences of the event, and therefore the level of risk, will also be largely influenced by the quality of
the source water and the effectiveness of pre filter treatment processes.
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
Ref P6.1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: PARTICLES NOT REMOVED cont’d
P6.1.1.3

Incorrect filter
backwash
procedure.

Determine which
parameter(s) will be
used to start the
backwash cycle, and
implement these
controls.
–
–
–
Time

Turbidity

Post-filter:
–
–
–
Aluminium
residual
concentration
Turbidity more
than 0.2 NTU
after the filter.

Frequent
backwashes.
Filter log.

Mud-balls
and/or cracks in
filters.

Boil-up and/or
dead spots
during
backwash.

Loss of media.
Change trigger
parameter for
backwash.

Change
procedure for
backwash
cycles.

Identify staff
training needs
and provide
training.
optimisation of the
filter ripening
operation.
Develop and use sitespecific pump
maintenance schedule.
Schedule should follow
manufacturer’s
recommendations and
include:
–

Turbidity

a log to record
identified problems,
and what was done
to rectify them

–
Frequent
elevated
turbidity in
filtered water.
turbidity criteria set
for filter-to-waste
protocol before
putting filter back in
service
Make sure operators
are trained in correct
maintenance and
operation procedures
for the specific filters in
use.
–

timing of air scour,
settling and water
wash

Inadequate
filter
maintenance.
Headloss

Determine and
implement effective
backwash cycle
procedure. Consider:
–
P6.1.1.4


Maintenance log
book (see also
P6.1.1.3).
(See also P6.3.1.3.) 
lubrication;
replacement cycle
for elastomeric
components (eg,
flexible couplings)
Increase
frequency of
media and filter
inspection if
excess residual
coagulant
detected in the
treated water.
component
inspection (valves,
pumps, compressors
etc)
inspection logbook.

Inspect and chemically
clean media at least
annually.

Start chemical
cleaning
programme.

Ensure operators are
trained in correct
maintenance
procedures for the
specific filters in use.

Identify staff
training needs
and provide
training.
Ref P6.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
5
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action
is needed
Event: NATURAL ORGANIC MATTER NOT REMOVED
Possible hazards: Germs; disinfection by-products (trihalomethanes, haloacetic acids, chloral hydrate (chlorine),
chlorite (chlorine dioxide), bromate, bromoform, formaldehyde (ozone)).
Level of risk: Moderate1
P6.1.2
The causes, preventive measures, etc., for this event are the same as those given for event P6.1.1. See
P6.1.1.1–P6.1.1.4.
Event: TREATMENT CHEMICALS CARRIED INTO DISTRIBUTION SYSTEM 2
Possible hazards: Health significant chemicals (acrylamide, epichlorohydrin).
Level of risk: Low.
P6.1.3
The causes, preventive measures, etc., for this event are the same as those given for event P6.1.1. See
P6.1.1.1–P6.1.1.4. See also Guide P5.3.1 which discusses use of treatment chemicals before filtration.
1
The consequences of the event, and therefore the level of risk, will also be largely influenced by the quality of
the source water and the effectiveness of pre filter treatment processes.
2
If treatment chemicals, either in soluble or insoluble form, are not removed by the filter they will pass into the
distribution system.
6
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
Ref P6.1
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 – Turbidity higher than maximum acceptable level
Indicators:

Turbidity less than 0.2 NTU cannot be maintained in the
water leaving the treatment plant.
Required actions:

Follow the actions given in Figure 3.4 of DWSNZ:2000

Identify the reason for the failure and rectify.

Record the reason for the failure and the steps taken to
rectify.

Modify the water safety plan if necessary.
Responsibility:
Manager designated responsible for water supply.
Event – Excessive levels of treatment chemicals carried into the distribution system
Indicators:
Required actions:
Responsibility:
Ref P6.1
Version 1, January 2014

Consumer complaints of discoloration, poor taste, sensation,
odour.

Excessive chemical concentrations measured in the water
leaving the treatment plant or in the distribution system.

Notification of error by operator or chemical delivery
contractor.

Follow Section 4.4 of DWSNZ:2000.

Identify the reason for the failure and rectify.

Monitor chemical until it reaches acceptable limits.

Record the reason for the failure and the steps taken to
rectify.

Modify the water safety plan if necessary.
Manager designated responsible for water supply.
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
7
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:

Turbidity, colour and aluminium in filtered water.

E. coli (faecal indicator) and coliforms.

Follow the protocols set out in DWSNZ:2000.
Note that the presence of faecal indicators may be influenced by factors other
than the adequacy of the filtration process.
How often:
What to do with the
results:
Responsibility:
8

For the monitoring frequencies of turbidity and E. coli DWSNZ:2000
Section 3.3.2.

Monitoring for colour and aluminium will likely be part of regular
process control, but as a minimum should be measured immediately
following a change in source water quality that requires a manual
change to chemical dosing.

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 rapid sand filtration.

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 rapid sand
filtration process are recognised in the plan.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Treatment Processes – Rapid Sand Filtration
Ref P6.1
Version 1, January 2014