Sustainable solutions for rural drinking water treatment

Rural communities often face high levels of contaminants, and water treatment systems which are more limited in design than large urban ones.

Treating drinking water in rural communities involves some very particular challenges. For one, rural areas commonly face water contamination by nitrates, as a result of fertilizers, septic systems, and manure storage or spreading. This often leads to nitrate concentrations that exceed the levels acceptable for drinking water standards.

But removing nitrates from drinking water presents challenges of its own in rural communities. Lacking the large, complex treatment systems of larger urban communities, rural areas require innovative, sustainable solutions to treat drinking water in an efficient manner.

Global drinking water quality guidelines for nitrates are <50 mg/L. However, some rural areas, such as in areas WSP examined in Western Australia, it is not uncommon to find nitrate concentration in rural communities between 70 and 130 mg/L.

Standard Methods

Nitrate removal treatment systems from groundwater resources typically fall under three technologies: Reverse Osmosis (RO), Electrodyalisis Reversal (EDR) and Ion Exchange (IX). All are capable of achieving global water quality guidelines for nitrate of <50 mg/L. As a case study, WSP experience in Western Australia had shown that some of these processes struggled to deliver a continuous water quality maximum for nitrate of <20 mg/L.

When reviewing groundwater quality in Western Australia, the standard form of treatment was reverse osmosis. However, with this process, the difference between the initial predicted nitrate levels and the end results were quite significant, and the recovery ratios were reduced, leading to higher volumes of waste.

A WSP study of the water quality in 93 rural communities uncovered some issues with the other two methods as well. The EDR process has a lack of competition because the process is only offered by one supplier in Australia, which made it difficult to get competitive prices because the equipment had to be imported as a complete plant. This process also involves complex operations and maintenance, expensive consumables and has encountered non-compliance issues in some areas.

With the IX process, the main concern was how the waste stream, which would have a high sodium chloride content, would affect the lagoon based sewage treatment process, which would receive that waste stream.

chart-Sustainable solutions for rural drinking water treatment-100

Estimation of Nitrate Treatment Technologies in operation in Australia

*IX process numbers based on point of use (POU) systems

New Methods

As part of the study, WSP also looked at new processes, particularly Biological De-nitrification (BD), which has been a common process in wastewater treatment but is something new in potable water treatment, with only few references in Europe and USA.

This process uses a bio-reactor, allowing bacteria to reduce nitrate to nitrogen. In some cases, the bio-reactors have gas exchange systems in place to remove the nitrogen while adding oxygen.
In principle, the biological process can offer:

  • Near 100% water recovery
  • Near zero sludge discharge

Various considerations are required to appropriately select the treatment systems for the removal of nitrate, especially for areas where the process will typically operate un-manned for long periods, such as six to eight weeks. There were two areas of interest in the biological approach: ease of operation and minimal volume of wastewater.

Water Recovery/Process Efficiency

To better understand the costs associated with a biological process to remove nitrates, we took actual operational values and compared them with the predicted biological values.

For the Reverse Osmosis process recovery, this was restricted to 60 per cent to ensure that silica scaling, which is an issue in the region, is minimized.

The table below shows the predicted flow recovery percentage, nitrate removal and waste volumes expected.

 

Nitrate, waste water and recovery ratio’s (to produce 550 m3/day with a raw silica level of 70 mg/l)

Unit RO* IX EDR BD
Groundwater Nitrate mg/l 120 120 120 120
Predicted Nitrate mg/l 40 8 40 <10
Produced Water Volume m3 550 550 550 550
Waste Water Volume m3 366 17 61 17
Process Efficiency % 60 97 90 97

*RO prediction results were checked against actual results from 3 plants operating on similar groundwater.

One of the common concerns in Australia against using an IX process is that it uses sodium chloride (NaCl) to regenerate the resin. In many cases what is not taken into consideration is that RO and EDR processes also reduce the total dissolved solids and when this is taken into consideration; the volume of dry salts for each process produced can be quite a different story.

Additionally, the waste streams result in a highly nitrate-contaminated solution, which can be a potential source of recontamination of the groundwater if not contained.

Capital and operational costs are key decision makers and when compared correctly can provide some interesting outcomes. For instance, in one case, RO was found to be the cheapest cost, but was actually the most expensive to operate.

The Biological Treatment Process

There was one challenge to overcome before a biological de-nitrification process could provide potable water, and that was to obtain the Western Australia Ministry of Health’s approval to supply the treated water to the potable water system.

Initial discussions with the Ministry of Health were held in 2009, to determine what protocols would be required and if there were any major concerns about such a biological process. The outcome was encouraging, and the Ministry of Health were involved in decisions on the first site to trial the process. The initial site (Blackstone) was chosen because of it was high quality water for the area, containing low total dissolved salts, a reasonable level of carbonaceous alkalinity and an average nitrate level of 70 mg/L. A second nearby site was chosen (Jameson) for the same reasons.

Tenders were received February 2010 and the order for the two townships, which when constructed would be the largest of their type in the world, even with a 350 m3/d capacity. The project was awarded in 2010 for two encapsulated biological nitrate systems.

Although both plants were constructed at the same time, commissioning was only started at Blackstone to allow lessons learned and changes could be replicated at Jameson prior to its start-up. The actual process commissioning commenced late in 2011 with the drinking water quality evaluation starting in 2012. Although the biological process had achieved a reduction in nitrates, there were a number of issues, such as increasing the nitrite values above the health limit, which further increased the test period. The original media supplier, who originally provided the plant sizing details, could not achieve an acceptable quality of drinking water. WSP was asked to assist the contractor during the second phase of testing and determined the retention time was not sufficient and the test methods were not fully understood.

After making several changes to the design, the system was tested, examined over a longer period, and WSP implemented changes to improve the process results.

The commissioning was completed in mid-2014, once set up and operating, the plant’s performance was monitored for six months. The performance report was completed and submitted to the Western Australia Ministry of Health and in 2015, the treated water was allowed to enter into supply.

The second plant at Jameson had been paused until the first plant had proven it could achieve the potable water quality and receive acceptance from the Ministry of Health. One of the issues for the Jameson plant was a requirement for new media, and due to the poor response from the original supplier, WSP was able to source an alternative supplier from Europe. Once the lessons learned from the Blackstone biological nitrate removal treatment plant were applied, the final water quality quickly came to the required levels. Jameson was able to provide potable water to the township in 2016.

REFERENCES

ASTM D3867 - Standard Test Methods for Nitrite-Nitrate in Water. 2009

Australian Drinking Water Guidelines 6 (NHMRC) 2011

Dordelmann, O, Panglisch, S, Klegral, F, Hell, F, Moshiri, A, Emami, A, Holl, WH, 2008, Nitrate Removal from Drinking Water in Iran – Assessment of Three Different Treatment Processes Based on Pilot-Scale Investigations.

EPA/600/R-11/040, April 2011, Arsenic and Nitrate Removal from Drinking Water by Ion Exchange. U.S. EPA Demonstration Project at Vale, OR Final Performance Evaluation Report.

Government of Western Australia, Department of Health, 2011, Nitrate in Drinking Water.

Leung E, 2013 Biological Treatment, Nitrate removal in drinking water. California Dept of Public Health, Drinking Water Program Seminar

Mohensi-Bandpi, A, Elliot, DJ, Zazouli, MA, 2013, Biological nitrate removal processes from drinking water supply – a review, Journal of Environmental Health Sciences & Engineering.

Persson, N, Jansen, JLC, Persson, KM, 2006, Biological Denitrification of drinking water.

Schoeman, JJ, 2009, Nitrate-nitrogen removal with small-scale reverse osmosis, electrodialysis and ion-exchange units in rural areas. Water SA Vol 35 No.5

Seidel, C, Gorman, C, Darby, J, Jensen, V 2012, Addressing Nitrate in California’s Drinking Water – Technical Report 6 Water Treatment for Nitrate with a focus on Tulare Lake Basin and Salinas Valley Groundwater. Prepared for California State Water Resources Control Board.

Seidel, C, Gorman, C, Darby, J, Jensen, V 2013, Nitrate Treatment Challenges – Ongoing Nitrate Treatment Studies. California Dept of Public Health, Drinking Water Program Seminar

Thorne, G 2014, Nitrate Removal Efficiency from Drinking Water an Assessment of Four Different Treatment Processes, Small Water and Wastewater Systems, Conference, Newcastle, NSW

Tredoux, G, Englebrecht, P, Israel, S, 2009, WRC Report No. TT 410/09 – Nitrate in Groundwater, why is it a hazard and how to control it?

Water Corporation, WA, 2010, Technology Assessment Report – Brackish water technology trials (Yalgoo & Wiluna WTPs).

WHO/SDE/WSH/04.08/56 – Rolling Revision of Water Guidelines for Water Quality, Treatment processes for Reducing Nitrite and Nitrate in Drinking Water – July 2004


More on this subject