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 Water Safety Plan Guide: iii
Version 1, January 2014 Filtration – Slow Sand Filtration
Ref P6.2 Water Safety Plan Guide: iii
Version 1, January 2014 Filtration – Slow Sand Filtration
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 NTU[1]).
· Pre-treatment (see the P1 and P4 series of Guides):
– processes removing particles, eg, sedimentation, can be used, so long as they do not require coagulation.
· 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:
– water coming into the filter must not disturb the sand
– the slower water passes through the filter, the better the water quality.
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.)
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 / 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
P6.2.1.1
Raw water quality too poor for the filter to treat. / · Monitor raw water quality to determine when the filter should be by-passed or treatment turned off. / · Raw water quality.
· Particle counts in water leaving the filter.
· Microscopic particle analysis of water leaving the filter.
· Volume of water passing through the filter before it needs to be cleaned – UFRV.2 / · Raw water turbidity is more than 20 NTU.
· Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
· UFRVs2 are unstable (indicates a possible problem with the way the filter is operating). / · Reassess whether this treatment is suitable for the typical raw water quality.
· Plan for source development or pre-treatment.
P6.2.1.2
Incorrect flow rates. / · Adjust inlet/outlet valves to balance flow rate according to the standard operating procedures for the filter. / · Flow rates. / · Flow rates are more than 0.2m/h.
· Head loss rate increases rapidly.
· Non-compliance with Section 3.2.3.1 of DWSNZ:2000. / · Review standard operating procedures.
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.
Causes / Preventive measures / Checking preventive measures / Corrective action /What to check / Signs that action is needed /
Event: PARTICLES NOT REMOVED cont’d
P6.2.1.3
Schmutzdecke3 is immature or ineffective. / · Protect filter from high turbidity waters and high levels of algae.
· Provide an adequate time after cleaning for the schmutzdecke to re-establish. / · Particle counts in water leaving the filter.
· Microscopic particle analysis of water leaving the filter.
· Filter headloss. / · Abnormally slow development of headloss.
· Non-compliance with Section 3.2.3.1 of DWSNZ:2000. / · 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.
P6.2.1.4
Incorrect or unsuitable cleaning procedures. / · Follow site-specific procedures for correct cleaning.
· Use protective boards when walking on the filter bed.
· Limit sand removal to the top 2–3cm. / · Measurement of sand depth removal pattern at regular intervals during cleaning process. / · Balance between inlet/outlet flows difficult to regulate.
· Abnormally long period after re-commissioning to achieve DWSNZ:2000 compliance. / · Review standard operating procedures.
P6.2.1.5
Development of oxygen-deficient conditions in the filter bed. / · 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).
· 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. / · Length of flow cycles through the two filter beds.
· Condition of filter beds. / · Non-compliance with Section 3.2.3.1 of DWSNZ:2000.
· Evidence of rotting matter in the sand.
· Unpleasant odours in the treated water.
· Greater use of one filter in the pair than the other. / · Review standard operating procedures.
· Ensure that water quality entering the filter is satisfactory.
3 The schmutzdecke is the mat of dirt, and living an dead organisms, on the top of the sand which removes particles and the germs.
Causes / Preventive measures / Checking preventive measures / Corrective action /What to check / Signs that action is needed /
Event: PARTICLES NOT REMOVED cont’d
P6.2.1.6
Poor quality assurance procedures. / · 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.
· Undertake regular audits of filter operating 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.
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.
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 removedIndicators: / · High turbidity of filtered water.
· More than 0.01% of 2–15 mm-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 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.
Responsibility: / · Manager designated responsible for the water supply.
Ref P6.2 Water Safety Plan Guide: 7
Version 1, January 2014 Treatment Processes – Slow sand filtration
[1] NTU nephelometric turbidity units.