CONTROL OF IRON BACTERIA PROBLEMS IN GROUNDWATER PUMPING SYSTEMS

 PETER FORWARD: BE (Elec., Hons.) South Australian Water Corporation, Berri, South Australia

 (Presented at "Irrigation Australia 96", The Irrigation Association of Australia Conference, Adelaide, May 1996)
 

1 INTRODUCTION
 The fouling of groundwater or drainage pumping systems by iron bacteria is a world wide problem resulting in impaired hydraulic performance, increased pumping costs, maintenance and corrosion. Naturally occurring bacteria (commonly called iron bacteria), given a suitable chemical and physical environment can thrive and through their metabolic processes convert soluble ferrous iron into its insoluble ferric form. The result is the familiar reddish-brown gelatinous mass of bacteria, iron hydroxides and water.

 Since the occurrence of iron bacteria is due to a combination of physical, chemical and microbiological factors, successful control measures must address all of these. Techniques developed by the South Australian Water Corporation have proven very effective. One method (now patented), employing in situ generation of chlorine by electrolysis of the groundwater being pumped, provides effective control of iron deposition, thereby eliminating this as a limiting factor in long term pump performance. This has wide application to groundwater pumping schemes in many areas which experience serious iron fouling problems and for which no simple economical long term solution has been found.

 The control techniques are discussed initially in the context of SA Water's responsibility for the operation of salinity mitigation schemes on behalf of the Murray-Darling Basin Commission (MDBC) which initially experienced severe problems with iron bacteria and therefore provided the catalyst for research and development of control measures.

2 BACKGROUND
 South Australia is vitally dependent on a guaranteed supply of good quality water from the River Murray. The natural resources of the Murray-Darling Basin are managed by the Murray-Darling Basin Commission and jointly funded by the Commonwealth and State Governments of New South Wales, South Australia and Victoria.

 The MDBC's Salinity and Drainage Strategy provides a framework for the four Governments to manage the pressing problems of salinity, water logging and land salinisation which are a serious threat to the economic and natural resources of the southern part of the Murray-Darling Basin. One of the elements of the initial program of works to reduce salinity was river salinity reduction schemes, one component of which is the Woolpunda Salt Interception Scheme.

 A substantial increase in River Murray salinity occurs within South Australia and by far the most significant area is in the Woolpunda reach between Overland Corner and Waikerie. The natural inflow of groundwater in this vicinity with a salt content of 20 000 mg per litre (two thirds of that of sea water) contributes 170 tonnes of salt per day, representing approximately 8% of the total salt load in the river.

 The Woolpunda Scheme is designed to intercept the groundwater flowing towards the river by pumping from 49 bores set in two lines either side of the river over a distance of 25 km. The pumps operate continuously delivering 20 ML per day to the Stockyard Plain Disposal Basin, 15 km south west of Waikerie. Less than twelve months after the commissioning of the scheme it was found that the performance of a number of pumps was deteriorating rapidly due to the buildup of iron hydroxide deposits within them. It was clear that the viability of the scheme was seriously threatened by the iron problem and that a solution was urgently needed.
 

3 OVERVIEW OF IRON BIOFOULING - CAUSES AND EFFECTS
3.1 Deterioration of Bores

 The major processes that cause deterioration of groundwater bores are; physical, chemical and microbial.

 The resultant problems affect parts of the system; bore, aquifer, pump and discharge components.

 In some cases some of the processes can simultaneously affect parts of the system to differing degrees. Recent studies provide excellent detailed material on the various mechanisms and rehabilitation techniques, McLaughlan et.al. (1), Borch et.al. (2), Howsam (3).

3.2 What is Iron Bacteria ?
 Iron bacteria (sometimes called "ochre" or "red water") is one aspect of the phenomenon of microbiological incrustation or biofouling of groundwater pumping systems which is now well recognised but still not yet fully understood. It is caused by the accumulation of microbes (bacteria), extracellular polymeric substances and inorganic precipitates (usually iron or manganese oxides). In the case of iron bacteria, they derive energy during their metabolism through oxidising soluble ferrous iron, present in the groundwater, to its insoluble ferric form. The resultant biofilm is commonly found as a slimy or gelatinous deposit on bore screens, pump inlets and internal waterways and discharge components, sometimes extending well into the distribution pipe system.

 In the absence of external contamination, aquifers were once assumed to be microbiologically clean. However, it is now known that bacteria are often endemic in aquifers throughout the world and that aquifers can allow the free transmission of bacteria under suitable conditions. Bores can also become contaminated from external sources such as infiltration of surface water and from drilling or maintenance equipment taken from bore to bore.

 The microbiology of iron bacteria is very complex and a number of chemical and physical factors are considered to be critically involved in assessing the risk of fouling in a bore, Cullimore (4). These include; aquifer characteristics, total iron, dissolved oxygen, temperature, pH and Eh (redox potential).

 Unfortunately, given the variability of interaction between these factors, it has not yet been possible to derive some form of "iron fouling potential index" to predict with certainty whether iron bacteria problems will be encountered at any particular site. Likewise, control measures used by operators vary widely in type and effectiveness and include a range of both physical and chemical techniques, often in combination. Their success is often dependent on the physical and operational aspects of the installation.

3.3 Factors Influencing Iron Bacteria Growth
3.3.1 Chemical Factors

 Iron bacteria have been found in many sites throughout the world with widely varying chemical conditions viz.: dissolved iron 0.01 to 10.0 ppm, pH 6 to 10, dissolved oxygen 0.5 to 4 ppm, temperature 4 to 25/ C and Eh +50mV to +800mV. Other factors include total phosphorous, dissolved organic carbon, total nitrogen and nitrate.

3.3.2 Physical Factors

 Many observers have reported that the thickest biofilms occur in the areas of highest water velocity such as pump inlets, bowls and discharge pipes and this has been the experience in numerous SA Water installations. It is of note that biofilms adhere extremely well to surfaces exposed to high velocities where they might otherwise be expected to be scoured away, even at velocities of up to 4 m/s, Tyrrel and Howsam (5 ).

There appears to be a physical relationship between water velocity and the rate of bacterial growth in that the higher the local velocity, the greater the supply of nutrients to the bacteria. Therefore the greater the pumping flow, the faster the rate of build up of bacteria in areas that experience high velocities such as inlet screens, pump internal water ways and discharge components. Conversely, reduce the flow and the rate of bacteria build up reduces, stop the pump and growth is minimal.
 

 This logic certainly seems reasonable in that in the remote aquifer the water velocity (and therefore nutrient flow) past any bacteria is very low, therefore growth rates are negligible. As the bore is approached, velocities increase and so do bacterial levels. In high flow situations, the critical velocities are high enough to cause sufficient bacterial growth in the aquifer and gravel pack surrounding the bore to cause clogging problems. In lower flow situations, these critical velocities are not reached until the pump inlet screen and within the pump where the high velocities and turbulence give rise to ideal physical conditions. Higher velocities, turbulence and pressure changes within the pump may also lead to gases coming out of solution with consequent pH changes etc. creating more favourable conditions for bacterial growth.

 It is postulated that these effects are due to the ease with which bacteria on the wall of a pipe or inside a pump are able to extract nutrients from the passing water. In a laminar flow situation in a pipe, theoretically the water velocity at the pipe wall is zero, therefore there is little supply of nutrients to the bacteria and their growth rate is limited. Under turbulent flow conditions however the pipe surface is continually washed with nutrients thus providing an ideal environment for the rapid growth of bacteria.

3.4 Effects of Iron Bacteria on Pumping Systems

 Depending on the severity of biofouling within a pumping system, various detrimental effects may occur including:

 * reduced hydraulic efficiency due to clogging of the gravel-pack, screen slots, pump and pipework.

 * corrosion of materials due to the creation of adverse electrochemical conditions.

 The severity of iron bacteria fouling varies widely from being merely found as a thin film of material deposited in parts of the system, rarely requiring attention, to extreme cases such as found in the Woolpunda and Waikerie Salt Interception Schemes in South Australia where 50 out of a total of 66 bores were affected. In the worst cases, a newly installed pump would lose 45% of its flow within 50 days of installation due to clogging of the internal waterways with iron deposits.
 

3.5 Rehabilitation of Bores
 

Many techniques are used to rehabilitate bores:
 

 * Physical methods include high pressure water jetting, compressed air surging and brushing or swabbing.
 

 * Chemical methods, often used in conjunction with physical agitation include the use of organic or inorganic acids in conjunction with surfactants and biocides such as chlorine, hydrogen peroxide and quaternary ammonium compounds. Many proprietary bore cleaning compounds are available using various combinations of these chemicals.

Borch et.al. (2) and Howsam (3) detail numerous rehabilitation techniques and case studies.
 

4 THE WOOLPUNDA SALT INTERCEPTION SCHEME
 

4.1 Investigations, Design and Commissioning
 

The designers of the Woolpunda Scheme were aware of the iron deposition problems that had been reported elsewhere but in the absence of some form of reliable "iron fouling potential index", were unable to be certain whether problems would ultimately be encountered at Woolpunda.
 

A total of seven pump test sites were used for detailed performance evaluation of both aquifer characteristics and pump types. Some tests showed the presence of iron deposits to varying degrees but none were considered serious.
 

During the first commissioning phase (Phase 1) the western-most 27 bores were pumped at double their ultimate flow rate for 6 to 8 months before the pumps were changed over for others to pump at the design flows (Phase 2). Whilst iron deposits were found in many of the original pumps, again it was to varying degrees but not to the extent that there had been any perceivable drop in pump output.
 

The design phase had allowed for the installation of a number of swabbing or "pigging" points to permit the insertion of foam rubber "pigs" to clean the pipelines should iron deposits occur in them. These were initially not installed due to their high cost but following the discovery iron deposits in the pumps and further experimentation on pig insertion methods, about half the points were eventually installed.
 

During the bore drilling, completion and pump installation stages, great care was taken to guard against contamination of bores (to prevent iron deposition problems) by chlorinating equipment, bores and pumps etc. However ultimately 42 of the 49 bores have developed iron problems with the other 7 having hydrogen sulphide present and no iron. It is therefore assumed that iron bacteria were endemic in the aquifer.
 

4.2 Investigations`Into the Iron Deposition Problems
 

Following installation of the design flow pumps in Phase 2, performance monitoring continued and it was soon evident that the flows from some pumps were beginning to reduce quite rapidly. This was naturally of great concern given that the initial 6 to 8 months operation was satisfactory. A number of pumps were removed for inspection which revealed serious clogging by iron deposits.
 

Subsequent monitoring of newly installed pumps revealed that in the worst cases, a pump would lose 45% of its output in as little as 50 days of continuous operation. Clearly a solution to the problem had to be quickly found.
 

 A number of research directions were suggested, mainly directed towards a better understanding of the chemistry and microbiology of iron biofouling. However from the scheme operators point of view what was needed was an immediate solution to the problem as the logistics of having to change pumps every three months or less was totally unacceptable.
 

 Since it was known that the iron deposition was of bacterial origin, it was postulated that chlorination of the pump on a regular basis might provide effective control. Furthermore, given that the groundwater had a high chloride content (typ. 12 000 mg/L) it was reasoned that there was potential to develop chlorine directly by electrolysis of the water being pumped.
 

In its simplest form, an electrolytic cell consists of two electrodes immersed in a conducting solution (electrolyte) and connected to a low voltage source of direct current (DC). As current flows through the electrolyte from the positive electrode (anode) to the negative (cathode), oxygen is released at the anode and hydrogen at the cathode. In the presence of a chloride electrolyte, the production of oxygen will still dominate unless the anode is coated with a catalyst of platinum group oxides (typically platinum, ruthenium, rhodium, osmium or iridium). If so, the catalyst will force chlorine to be produced preferentially. This is the principle of the common "salt water" swimming pool chlorinator which typically uses titanium electrodes, the anode of which has a catalytic coating, arranged in either two concentric cylinders or in parallel plates.
 

Initial ideas envisaged an electrode assembly surrounding the pump inlet screen but the first trials carried out at the surface quickly indicated the impracticability of this arrangement. These demonstrated that at an electrode current of 25 Amps DC, about 1.0 ppm of free chlorine could be developed at a water flow of 4.0 L/s. It was observed that if the current were reduced to 20A no chlorine was detectable. This was understandable given the typical dissolved iron levels of 1.5 mg/L in that any chlorine initially produced would immediately react with the iron and only once this demand was satisfied would any excess chloride be detected. Thus to produce a chlorine residual of 4.0 ppm, an electrode current of 35 to 40 Amps would be needed. This in turn would have required a substantial power supply at the surface and heavy cables running down to the electrode assembly which would have needed to be quite large due to the requirement to keep within the rated current density (Amps/m²) of the electrode material.
 

Another problem to be overcome was that of carbonate deposition on the cathode. In a normal salt water swimming pool there are low levels of bicarbonate in solution and these gradually build up on the cathode as carbonate, thereby reducing the current flow and therefore chlorine production. Periodically these must be removed by cleaning with high pressure water or with dilute acid. At Woolpunda however with a high level of bicarbonate in the water (typ. 500mg/L), the initial trial electrode assembly turned into a solid cylinder of carbonate within a few days. These problems have now been overcome with a special type of electrode material and power supply.
 

4.3 Implementation of a Successful Chlorination System
Given the problems of locating the electrode assembly at the pump inlet and chlorinating the normal pump flow, an alternative method (now implemented) was developed as shown in Figure 1. In this arrangement the chlorinator is still located in the pipe carrying the full pumped flow but is normally not energised. Once a day the pump is stopped for an hour, at which time the main reflux (check) valve closes preventing uncontrolled flow from the system back down to the pump. However for this period the chlorinator electrode is energised at 10 Amps DC and a flow of 9 litres per minute, as regulated by the "Maric" flow controller, allowed to bypass the reflux valve and flow back down the riser hose. This water with a free chlorine level of 3 to 4 ppm has been found to adequately disinfect the pump. The adopted method is a compromise between the down time period of the pump, the size and therefore cost of the power supply and electrode assembly and the chlorine level required to ensure effectiveness.
 

One of the early trials was conducted on a pump which had already deteriorated from an initial flow of 4.1 L/s to 2.0 L/s. Back flushing at 4.0 ppm free chlorine did not have any immediate beneficial effect so it was back flushed for 2 hours, twice a day with a chlorine concentration of 100 ppm. This high concentration was achieved by reducing the back flow to 2 litres per minute and using an electrode current of 25 Amps. This had some effect in gradually improving the flow to 2.5 L/s over 60 days presumably by slowly scouring away some of the iron deposits (and as it turned out later, badly pitting the pump). Whilst this chlorination could not quickly remove all the iron deposits, it did however prove that the intermittent disinfection of the pump prevented the further accumulation of iron deposits. Subsequently the chlorine level was reduced to 4.0 ppm and continued to be effective, this level now being adopted for all 42 iron affected bores. T>

Figure 1: Typical electrolytic chlorinator arrangement

 The results of early trials are shown in Figure 2. Both bores 24 and 47 showed similar rapid reductions in output due to the accumulation of iron deposits. After the installation of chlorinators, performance was maintained until in the case of bore 47 the chlorinator was turned off after a while whereupon its performance deteriorated at a similar rate to that previously.

Figure 2: Before and after chlorination performance of pumps

4.4 Long Term Performance of Chlorination Systems
 

 After extensive operational experience since late 1991, it can confidently be said that iron biofouling problems within the pumps are totally controlled in the 42 bores that have iron bacteria present. Many of the pumps in these bores have now operated continuously for more than 2 years since the last changeover with the longest having operated for 3 years 3 months with no loss of output attributable to iron biofouling. This pump was removed for no other reason than it was desired to change its flow rate. Upon dismantling, it was found that the internal waterways were still free of iron deposits although there were some present in the "dead" areas of the impeller bowls (i.e. parts through which water does not normally flow and which do not get adequately disinfected during backflushing) but this had no effect on pump performance. This pump was one of the previous extreme cases with 45% loss in output within 50 days of installation.

4.5 Matters of Interest to Other Operators Suffering Iron Biofouling Problems
4.5.1 Bore Equipment, Construction, Operation and Maintenance
 

In designing the Woolpunda Scheme and being aware of the corrosive nature of the groundwater, a number of decisions were made which have also considerably assisted with coping with the iron biofouling problem.
 

All materials chosen were corrosion resistant such as the fibreglass and PVC bore casings and screens, grade 316 or 904L stainless steel pumps, motors, surface pipework and valves and flexible "Wellmaster" or "Foraduc" textile reinforced riser hose, also with stainless steel couplings. A further benefit of using such materials has been the ability to easily clean a pump in situ should it become blocked due to an undetected chlorinator fault. In these situations it has been found that stopping the pump and pouring 10 to 15 litres of 60% nitric acid directly down the riser column and leaving it for 2 to 3 hours before restarting the pump, effectively removes the built-up iron deposits. Sometimes a second dosing is required to fully restore the pump's performance.
 

Whilst the chlorination system keeps the pumps free of iron, some deposition still occurs in the pipework at the surface in the vicinity of the bore and this has to be pigged every six months in order to obtain accurate flow measurements using a clamp-on ultrasonic flowmeter.
 

There also appears to be an advantage in using a flexible riser column in that whenever the pump stops, water drains back down through the pump, collapsing the hose and dislodging any iron deposits within it. If a solid riser pipe were used it would probably rapidly block up.
 

After up to six years of pumping there is no evidence of reduced hydraulic performance of any of the bores which would be a sign of iron fouling of the gravel pack and the areas adjacent to it. The literature contains many case studies where this has occurred but these would generally seem to be in situations of high pumping rates (greater than 10 L/s) and relatively short lengths of slotted casing. Iron biofouling seems to be worst in areas of high velocity and in these situations water velocities in the gravel pack and through the screen slots appear to be high enough to cause problems. In the Woolpunda Scheme, pump flows are in the range of 1.5 to 10.0 L/s and the slotted casing lengths are approximately 50 metres so the problem of iron deposition may remain confined to only affecting the pumps in the absence of chlorination.
 

4.5.2 Iron Deposition in Pipelines
Interestingly, iron deposition does not generally appear to extend to any great degree beyond the spur mains that connect each bore with one of the two collection mains with few deposits of any consequence having been found in the latter. Significant deposits have however been found in the last 3.5km of the disposal main to Stockyard Plain Basin which have affected its hydraulic capacity. These are thought to be exacerbated by further aeration and agitation of the water as the pipe runs only partially full for much of this distance. In addition, entrapment of air within this section of main necessitated the installation of additional air vents to reduce hydraulic losses. The iron bacteria deposits however have been successfully removed by regular pigging.
 

Although it has not yet been confirmed, there would appear to be no reason why the same chlorination system used for iron bacteria control could not be used for the control of other bacterially induced problems such as hydrogen sulphide and the corrosion problems that arise from it. Trials have commenced on some bores but the pumps have not yet been removed for examination.

5 WAIKERIE SALT INTERCEPTION SCHEME
 

 The Waikerie scheme is located almost immediately downstream (2 km) of the Woolpunda scheme and consists of 17 bores intercepting saline groundwater being forced into the River Murray due the presence of a large groundwater mound caused by drainage water from the Waikerie Irrigation District. The bore flows vary from 4.5 to 10 L/s with a total of 12 ML per day being pumped to Stockyard Plain Basin. The scheme was constructed immediately following Woolpunda and uses the same basic configuration of pumps, riser columns and materials with a few changes implemented, based on operational experience to that time.

 As is typical of the geographical variability of the iron bacteria problem, of the nine eastern most Waikerie bores nearest Woolpunda, only one has an iron bacteria problem and the other eight are totally clean with no deposition of any kind. This is in contrast to Woolpunda where the seven bores that do not have iron bacteria have hydrogen sulphide present with these all being in the same location with four on one side of the river and three immediately opposite on the other side. Presumably this is due to some local geological feature more conducive to the growth of sulphate reducing bacteria rather than iron bacteria.
 

 The western eight bores all developed iron bacteria problems, some even more severe than Woolpunda, with heavy deposition evident within a few months of the commencement of pumping. Chlorination systems were installed on these bores and have successfully eliminated any problems within the pumps.
 

6 RUFUS RIVER SALT INTERCEPTION SCHEME
 

The Rufus River Scheme was commissioned in 1984 to intercept saline water being forced into Rufus River by the hydraulic influence of the adjacent elevated Lake Victoria Storage. It consists of four lines of well point spears with four conventional centrifugal pumps operating in a suction lift configuration with 40 to 50 spears on each of the lines and a total of 178 spears in the scheme. Although soon after pumping commenced, iron bacteria deposits had been found in the discharge of the well point pumps and discharge pipelines (which had to be regularly pigged), no deposits had been found on the suction side of the pumps or in the vacuum tanks. The pumps were still delivering near their required flows so there was no immediate evidence of any problem with the spears themselves.

 As part of an investigation into enhancing the performance of the scheme, in February 1995 a number of spears on Line 3 were excavated to connect them to a trial air lift pumping system. It was found that the spears and the flexible lines connecting them to the suction manifold were badly blocked with iron bacteria deposits. When constructed, each spear yielded between 2 and 3 L/s, however some were found to be yielding less than 0.05 L/s.
 

 Work commenced to clear these blockages and modify the spears and flexible connections to the suction manifold to enable future testing and cleaning of individual spears. At this stage the condition of the remaining spears in Line 3 was unknown and likewise those in Line 4, a short distance away, also on the eastern side of Rufus River and Lines 1 and 2 on the western side.
 

Almost every spear on Line 3 was eventually found to be badly affected, although a few were clear. Those that were clear were found to be yielding significantly above the design flow of 0.4 L/s thus making it appear (from the flowmeter on the pump) that the whole line was performing satisfactorily. Unfortunately the monitoring of groundwater levels in the area had not been detailed enough to indicate that many spears were not performing.
 

 Subsequently most of the remaining spears on Line 4 were also found to be almost totally blocked with iron deposits and the suction manifolds both there and on Line 3 were partially blocked therefore requiring the installation of pig (swab) insertion and exit points to enable them to be cleaned.
 

Following the completion of work on Line 4, it continued on to Line 2 where again blocking of the spears and flexible connections was found. Subsequent work revealed that many spears and flexible connections in Lines 1 and 2 were also blocked to varying degrees thus requiring similar rehabilitation to that carried out for Lines 3 and 4.
 

The blocked spears and flexible lines have been successfully cleared by compressed air scouring and yields have been restored. Stainless steel ball valves have been installed on each spear to connect to the flexible pipe as well as on the top of each spear to enable them to be flow and vacuum checked in the future and cleaned if required.
 

A more detailed monitoring strategy is being formulated to regularly monitor pump and pipeline hydraulic performance, spear yields and groundwater levels to ensure the wellpoint lines operate at maximum efficiency. It is planned to install chlorination systems on all well point lines in a similar manner to that used to control iron bacteria at Woolpunda and Waikerie and a small scale pilot system is presently being evaluated.
 

7 RUFUS RIVER AIR LIFT SYSTEM TRIAL
 

 Salt load monitoring in Rufus River appeared to indicate that the Rufus River Scheme was still not intercepting a significant amount of the salt entering the river. In early 1995 it was proposed to trial an air lift system as a means of enhancing the scheme, as an extension of and an alternative to enlarging the existing conventional well point system.

 Given the problems already experienced with iron bacteria in the area, doubts were held about the susceptibility of an air lift system to clogging with deposits, particularly as a plentiful supply of oxygen encourages bacterial growth. These concerns were vindicated when after 30 to 50 days the yields from the spears were observed to be falling. Inspection revealed a mild buildup of iron deposits and after 100 days this had severely blocked the injector tubes.

 Whilst the air lift system performed well in terms of the initial flow requirements, the eventual iron bacteria deposition resulted in the trial being abandoned. It may be possible to control the bacteria through chlorine dosing and trials will commence if an air lift pumping system looks, in all other respects, to be a viable way of enhancing the scheme.

8 SYSTEM DESIGN TO TOLERATE IRON BIOFOULING

 Iron bacteria are widely found in Australia but likewise also absent in many other locations. Local knowledge from other pump owners is the best guide but from this point it is very difficult to predict just how severe the problem will be for any particular installation. In some instances it has been found in one bore but not in another literally tens of metres away.

 Based on SA Water experience and that gleaned from numerous accounts from operators world wide, there are a number of factors that prudent system designers should take into account or ignore at their peril. A rule to remember is that "An iron bacteria problem only gets worse", unless control measures are implemented.

8.1 Bore Construction

 * Take great care during drilling and installation to disinfect all equipment to prevent contamination being introduced from elsewhere. The bacteria may well be endemic in an area and an iron problem may arise anyway but these precautions are easy to implement and indeed mandatory in many areas.

 * Use a non corroding casing (eg. PVC or fibreglass) which will ensure longer life and not be affected by any bore cleaning chemicals (often acidic) that may be needed later.
 

 * Ensure the casing is sealed to prevent contamination from surface water ingress.

 * The slotted casing should be as long as practicable to provide the maximum inlet area and the slot widths as large as possible, thereby minimising local water velocities and turbulence which are known to promote bacterial growth.
 

8.2 Pump
* Made of corrosion resistant materials for the water being pumped but more importantly able to withstand any cleaning chemicals that may be used in cases where the iron fouling is confined mainly to the pump and it is possible to clean it without having to remove it.

* Constructed such that it is easy to remove, dismantle, clean and reassemble.
 

* The pump flow should be as low as possible, commensurate with the daily volume required but recognising that greater extraction rates will result in higher local water velocities and turbulence, thereby increasing the rate of bacterial growth in those areas.

 * Minimise the draw down in the bore by pumping at the lowest practicable flow rate. This will in turn reduce the entrainment of air into the water and keep the water surface further above the pump, making less air available to the bacteria and so reduce their growth rate.

8.3 Discharge Pipework
* The discharge pipework should be easily removable or other provision made for dosing of bore cleaning chemicals if required.
 

* Pipework and fittings should be of larger diameters to minimise turbulence and allow for easy dismantling for cleaning.
 * Provision should be made for insertion of foam rubber pigs (swabs) to clean pipelines. These points should be located at changes in pipe size and usually comprise an A branch, isolating valve, extension barrel to hold the pig, with an access flange and valve through which water is pumped to launch the pig.
 

8.4 Pipeline System

If there is any possibility of an iron deposition problem occurring, careful attention must be paid to the design of the pipeline system to maintain efficiency and minimise ongoing power and maintenance costs.
 

* Be conservative in sizing pipelines, particularly up to 150 or 200mm. It is strongly recommended that the next larger size be selected to minimise the adverse hydraulic effects of iron deposits. For example in one case in the Woolpunda Scheme, the buildup of iron deposits over 14 months in a 100mm pipe with a flow of 7 L/s, resulted in a head loss of 16 metres over only 150 metres of pipe. In another case after 16 months with a flow of 5 L/s, there was a head loss of 40 metres over 220 metres of 100mm pipe. Remember that the rate of iron deposition is largely velocity dependent.
 

 * Provide pig insertion and exit points at convenient locations.

 * Carry out regular pressure checks along the pipeline to detect extra head losses due to iron build up and schedule pigging programs accordingly.

 * Make sure that the pumps are not pumping air into the system which will accelerate iron bacteria growth as well as accumulating at physical and hydraulic high points, thereby increasing head losses. Use quality air release valves of sufficient discharge capacity and take care with the grading of pipes and location of air valves.

9 PUMP AND BORE PERFORMANCE MONITORING
 Having designed and installed a system to minimise potential iron fouling problems, it is essential that regular monitoring is carried out to detect any loss of performance due to pump wear or iron fouling. The following is required:

 * A discharge pressure gauge.

 * A suitable flowmeter. In many cases however this too will become clogged by iron deposits and sometimes tend to begin reading fast at the same time the pump flow is actually dropping off, thus masking the fact that any adverse effects are occurring! A better proposition is to use a flowmeter that can be put in line as required or use a clamp-on ultrasonic flowmeter, providing the pipework has been pigged immediately prior to testing to remove iron deposits which will otherwise affect its accuracy.
 

 * A means of measuring the dynamic water level in the bore such as an air line, electronic sensor or measuring tape with a buzzer.
 

 By measuring the above parameters it is easy to plot the current pump performance on the pump curve and compare this with that measured after installation. This readily highlights deterioration in pump performance and allows corrective action to be taken promptly.

 Similarly, regular comparison of the draw down levels in the bore will detect the situation where this begins to decrease with little change in flow, indicating that the screen slots or gravel pack are becoming blocked with iron deposits.

10 CONCLUSIONS

 The problems caused by iron biofouling can range from the minimal to the intolerable. In the case of the Woolpunda Salt Interception Scheme, its operational viability was seriously threatened until the electrolytic chlorination system was developed. The daily back flushing of the pumps with a low level chlorine solution provides sufficient disinfection to prevent the formation of a biofilm within them. This appears to have completely controlled the problem within the pumps with some having now run continuously for more than 3 years with no performance deterioration attributable to iron biofouling. Extension of these control measures has had similar success in the Waikerie Scheme and shows potential for the Rufus River Scheme.

 Experience in recent years within SA Water has highlighted the need for careful attention to initial system design to minimise the effects of iron bacteria. Equally important however is the development and rigorous implementation of ongoing performance monitoring and maintenance strategies to minimise maintenance costs and maximise system efficiency and availability.

 There is still much to be learnt about the iron bacteria problem and it is essential that operators continue to develop control techniques and exchange knowledge in a cooperative manner.

REFERENCES

 1. MCLAUGHLAN R.G. AND STUETZ R. Fouling and Corrosion of Groundwater Wells: A Research Study, National Centre for Groundwater Management, University of Technology, Sydney, 1993.

 2. BORCH M.A., SMITH S.A., NOBLE L.N. Evaluation and Restoration of Water Supply Wells, AWWA Research Foundation and American Water Works Association, 1993.

 3. HOWSAM P. Water Wells: Monitoring, Maintenance, Rehabilitation. Proc. of the International Groundwater Engineering Conference. P. Howsam (ed.) E.&F.N. Spon, London, 1990.

 4. CULLIMORE D.R. (Ed.) Physio-Chemical Factors in Influencing the Biofouling of Groundwater. Proc. of the 1986 International Symposium on BiofouledAquifers: Prevention and Restoration. D.R. Cullimore (ed.), American Water Works Association, 1987, 23-36.

 5. TYRREL S.F., HOWSAM P. Monitoring and Prevention of Iron Biofouling in Groundwater Abstraction Systems; Water Wells, Monitoring, Maintenance, Rehabilitation. Proc. of the International Groundwater Engineering Conference. P. Howsam (ed.) E.& F.N. Spon, London 1990.

 6 FORWARD P.D. Control of Iron Biofouling in Submersible Pumps in the Woolpunda Salt Interception Scheme in South Australia.Proc. of Water Down Under 94, The Institution of Engineers, Australia, 1994, Vol. 2, 169-174.