Water Safety Plan
Guide
Filtration –
Slow Sand Filtration
Version 1, Ref P6.2
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide: Filtration
– Slow Sand Filtration, Version 1, ref p6.2. Published in January
2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42732-5 (print)
ISBN: 978-0-478-42733-2 (online)
Previously published in 2002 as Public Health Risk Management
Plan Guide: Filtration – Slow Sand Filtration, Version 1, ref p6.2. 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
6
Water Safety Plan Performance Assessment
7
Ref P6.2
Version 1, January 2014
Water Safety Plan Guide:
Filtration – Slow Sand Filtration
iii
Introduction
Slow sand filtration can be used to remove germs, particles and natural organic matter from
water. These contaminants are the concern of this Guide. Slow sand filtration can also
remove compounds that cause tastes and odours, but these are not considered here.
Slow sand filters are simple to construct and operate, and do not require the use of chemicals.
The top layer of the filter, which contains dirt, and living and dead organisms, is mainly
responsible for removing contaminants from the water. Slow sand filters have been shown
to be good at removing germs, including Giardia cysts. They are poor at removing colour
from water. It is important to disinfect the filtered water before use, however, because some
of the organisms living in the sand may be carried into the filtered water.
If an event occurs during the operation of the slow sand filter (ie, the process doesn’t 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.
Slow sand 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 quality of the water produced by slow sand filtration will affect how well any following
disinfection works. How well the filters work depends on:

Raw water quality (see Guide S1.1):
–
slow sand filters are easily clogged and need raw waters with low concentrations
of algae and other particles (turbidities less than 15-20 NTU1).

Pre-treatment (see the P1 and P4 series of Guides):
–


Filter operation:
–
–
filters have to be cleaned from time to time by scraping off the top layer
after cleaning, water quality may not be at its best for several hours or days.
This is the time needed for living organisms to grow on top of the filter again
–
the filter works best after its been cleaned a few times.
Filter design:
–
–
1
processes removing particles, eg, sedimentation, can be used, so long as they do
not require coagulation.
water coming into the filter must not disturb the sand
the slower water passes through the filter, the better the water quality.
NTU nephelometric turbidity units.
Ref P6.2
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Slow sand filtration
1
Risk Summary
The event creating the greatest risk involved in slow sand filtration is poor removal of
particles (see P6.2.1).
The most important preventive measures are:

to make sure the raw water quality is suitable for the filter (see P6.2.1.1)

to carry out cleaning of the filter correctly (see P6.2.1.4).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Filtration – Slow Sand Filtration
Ref P6.2
Version 1, January 2014
Risk Information Table
Reliable information about raw 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
What to check
Corrective action
Signs that action is
needed
Event: PARTICLES NOT REMOVED
Possible hazards: Germs not removed (particularly Giardia and Cryptosporidium).
Level of risk: High1
P6.2.1.1

Raw water
quality too poor
for the filter to
treat.
P6.2.1.2
Incorrect flow
rates.

Monitor raw water

quality to determine
when the filter

should be bypassed or treatment
turned off.

Adjust inlet/outlet
valves to balance
flow rate according
to the standard
operating
procedures for the
filter.
Raw water
quality.
Particle counts
in water leaving
the filter.

Raw water
turbidity is more
than 20 NTU.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Microscopic
particle analysis
of water leaving 
the filter.

Volume of water
passing through
the filter before
it needs to be
cleaned –
UFRV.2

Flow rates.

Reassess
whether this
treatment is
suitable for the
typical raw water
quality.

Plan for source
development or
pre-treatment.

Review standard
operating
procedures.
UFRVs2 are
unstable
(indicates a
possible problem
with the way the
filter is operating).

Flow rates are
more than
0.2 m/h.

Head loss rate
increases rapidly.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
1
The consequence 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 slow sand filtration.
2
Unit Filter Run Volume.
Ref P6.2
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Slow sand filtration
3
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: PARTICLES NOT REMOVED cont’d

P6.2.1.3
Schmutzdecke3
is immature or
ineffective.

P6.2.1.4
Incorrect or
unsuitable
cleaning
procedures.
P6.2.1.5

Follow site-specific
procedures for
correct cleaning.

Use protective
boards when
walking on the filter
bed.
Limit sand removal
to the top 2–3 cm.

When operating

filters in tandem
ensure that
alternation of the
flow between the

two filters is
balanced (to avoid
organic matter
penetrating the filter
bed and rotting).


4


Development of
oxygendeficient
conditions in the
filter bed.
3
Protect filter from

high turbidity waters
and high levels of
algae.

Provide an
adequate time after
cleaning for the
schmutzdecke to re
establish.

Abnormally slow
development of
headloss.
Microscopic

particle analysis
of water leaving
the filter.
Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Particle counts
in water leaving
the filter.

Reassess
whether this
treatment is
suitable for the
typical raw water
quality.

Assess how filter
can be better
protected from
poor water
quality, and
implement
changes.

Review standard
operating
procedures.

Change to
alternate filter and
increase ripening
period of original
filter.

Review standard
operating
procedures.

Review standard
operating
procedures.

Ensure that water
quality entering
the filter is
satisfactory.
Filter headloss.
Measurement of 
sand depth
removal pattern
at regular
intervals during

cleaning
process.
Balance between
inlet/outlet flows
difficult to
regulate.
Abnormally long
period after recommissioning to
achieve
DWSNZ:2000
compliance.
Length of flow
cycles through
the two filter
beds.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.
Condition of
filter beds.

Evidence of
rotting matter in
the sand.

Unpleasant
odours in the
treated water.

Greater use of
one filter in the
pair than the
other.
Ensure newly
matured beds
receive a regular
flow of water to
maintain their
biological
characteristics.
Avoid build up of
biological (eg,
algae)and
particulate matter in
the filter bed.
The schmutzdecke is the mat of dirt, and living an dead organisms, on the top of the sand which removes
particles and the germs.
Water Safety Plan Guide:
Filtration – Slow Sand Filtration
Ref P6.2
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: PARTICLES NOT REMOVED cont’d
P6.2.1.6

Ensure sampling
procedures comply
with the
requirements of
DWSNZ:2000.

Provide staff
training in:
– operation of
slow sand filters
– collection of
monitoring
samples
– recording of
monitoring
results.
Poor quality
assurance
procedures.


Monitoring as
per Section
3.2.3.1
DWSNZ:2000.

Non-compliance
with Section
3.2.3.1 of
DWSNZ:2000.

Identify
shortcomings in
sampling
procedures and
rectify.

Identify
shortcomings in
staff training and
rectify.

Determine
whether changes
to operating
procedures need
to be made.
Undertake regular
audits of filter
operating
procedures.
Event: NATURAL ORGANIC MATTER NOT REMOVED
Possible hazards: Disinfection by-products (trihalomethanes, haloacetic acids, chloral hydrate (chlorine) and
chlorite (chlorine dioxide)), and germs not removed.
Level of risk: Low4
P6.2.2
The causes, preventive measures, etc., for this event are the same as those given for event P6.2.1. See
P6.2.1.1–P6.2.1.6.
4
The consequences of the event, and therefore the risk, will be influenced by the quality of the water source.
Ref P6.2
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Slow sand filtration
5
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:
6

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 – Slow Sand Filtration
Ref P6.2
Version 1, January 2014
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:
Ref P6.2
Version 1, January 2014

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 slow sand 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 slow sand filter are recognised
in the plan.

Manager designated responsible for the water supply.
Water Safety Plan Guide:
Treatment Processes – Slow sand filtration
7