producing high quality drinking water from mine impacted ... · standards for drinking water...

275Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)

Producing High Quality Drinking Water From Mine Imapcted Water At High Recoveries Using

Reverse Osmosis MembranesAndrew Hammond1

1 Aveng Water, 1 Jurgens Road, Jet Park, Aveng Park, Gauteng, 1459, South Africa, Tel: +27 10 205 1800 [email protected]

Abstract� ree Aveng Water designed HiPRO™ plants have been operating for a number of years. Each of these plants produce a product water that complies with the South African standards for drinking water (SANS241), two of which are currently supplying drink-ing water to the community. Key to producing this quality of water is the use of Reverse Osmosis Membranes. Management of these membranes is essential to maintaining con-sistent and reliable supply of water from the plant. � e RO membranes on these plants are sensitive to inorganic scaling and biological/organic fouling due to the process and the feed water being reclaimed. � e performance of these membranes is shown. � e annual Salt Passage increase on the RO membranes is higher than the initial design but still within acceptable limits for potable water. A highly technical team is required to sustainably operate a plant of this nature. Keywords: ICARD | IMWA | HiPRO™ | RO Membranes | Mine Water Treatment

Introduction Water scarcity in South Africa and much of the world is becoming more pervasive. With a de� cit in supply and demand of conventional sources, it is going to become paramount that alternate water sources are investigated as a way to augment the supply. It is estimated that there will be a shortfall of 83.3 Ml/day in the region around eMalahleni Mpumulang by 2030 (Coleman et al., 2011). Mine water reclamation presents one such augmentation strategy. It has estimated that mining activi-ties in the upper Oliphants catchment area has created a need to treat between 200 and 250 Ml/day of Mine Impacted Water. � ese waters are complex and characterised by high levels of sulphate and a combination of bal-ancing ions (Calcium, Magnesium, Acidity and monovalents) in various proportions, de-pending on the source. � e resulting waters require rigorous treatment to remedy. One such active treatment process that can be used to remedy the mine impacted waters and pro-duce potable water is Aveng Waters HiPRO™ process. � is process combines chemical precipitation with advanced membrane pro-cesses allowing for large scale treatment of mine impacted waters. Currently four major

installations use this technology and were all funded and owned by the senior coal miners in the Mpumalanga province. � e four plants are the eMalahleni Water Reclamation Plant (EWRP) – (Phase 1 and Phase II), � e Op-timum Water Reclamation Plant (OWRP) and the Middleburg Water Reclamation Plant (MWRP). � ese four plants each are able to produce a product that complies with the regulatory requirements as speci� ed by SANS241. Currently OWRP and EWRP pro-duce water for the surrounding community.

Feed Water Quality Table 1 summarises the design capacity of each plant, the year of commissioning, the plant owner, the plant designer and the typi-cal feed water quality (major components only) currently being processed by the plant. It can be seen that the feed water quality dif-fers between plants.

HiPRO™ Process A detailed explanation of the process can be found Hutton et al (2009) so it will not be covered here, however Figure 1 shows the ba-sic block � ow diagram of the process.

� e RO sections of the plant are the last

6_Acceptability of Processed Mine water BOOK.indb 275 9/3/18 11:41 AM

11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”

276 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)

step in each stage of the process. Once an ul-tralow suspended solids water is produced from the UF membranes, the RO membranes desalinate the water and produce a low TDS product and a moderately high TDS brine. � e brine from the RO then proceeds to the precipitation reactors. In these reactors, the salinity is reduced through chemical precipi-tation. Due the nature of the feed water, the main aim of the reactors is the sequential re-moval of sulphate from the feed. � e e� ect of the inter-stage precipitation on the removal

of sulphate is pronounced and can be seen in Figure 2.

When looking at the typical concentra-tions of sulphate through the plants (Figure 2) of EWRP Phase I, OWRP and MWRP, it can be clearly seen how e� cient the precipita-tion steps are at removing sulphate. � e Stage 2 precipitation removes the largest mass of sulphate relative to the feed (~70%) and the Stage 3 precipitation removes ~12.5% rela-tive to the feed. � e monovalents present in the feed water that cannot be precipitated

Table 1: Summary of Major Aspects of the plant installations

EWRP Phase I OWRP MWRP

Plant Owner Anglo American Thermal Coal

Optimum Colliery South 32

Plant Designer Aveng Water Aveng Water Aveng Water

Year Commissioned 2008 2011 2015

Design Capacity 25 Ml/day 15 Ml/day 25 Ml/day

Feed Water -Typical

Calcium (mg/L) 520 414 460

Sulphate (mg/L) 2500 3000 3400

Magnesium (mg/L) 180 450 650

Sodium 130 160 60

Acidity (mg/L as CC) 80 0 0

Alkalinity (mg/L as CC) 10 180 180

pH 6 8.0 7.5

TDS (mg/L) 3500 5021.6 5000

Figure 1: Basic Process Flow of HiPRO™

STAGE 3:

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Mine Feed Stage 1 Precipitation

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Stage 1 Reverse Osmosis

Stage 2 Precipitation

Stage 2 Ultrafiltration


Stage 3 Precipitation

Stage 3Ultrafiltration


Product

Brine To Pond

STAGE 1:

Mixed SludgeTo Pond

High Purity Gypsum




are either permeated via the RO membranes or concentrated through the plant and dis-charged as a high salinity � nal reject stream. � e secondary bene� t of the staged precipita-tion is the recovery of water while minimis-ing the required electrical power. � e combi-nation of targets to save energy on one hand and provide a well-controlled water quality on the other has been reached by employing this staged precipitation and high-rejection low energy RO elements. � e average plant power consumption per m3 of product ranges between 1.8 and 2.0 kWh/m3 while achieving water recoveries between 97% and 99% (wa-ter quality dependent).

Sustainable RO OperationAs with any membrane installation, the con-trol and measurement of the feed water qual-ity is paramount in maintaining sustainable operation. In these particular waters, this can be broadly be broken into control of the scal-ing potential and control of fouling.

Scaling ControlAntiscalant dosing and control is of essential importance in controlling scale formation within each stage of the process. Preventing the formation of calcium sulphate crystal for-mation on the membrane surface, key water quality parameters are measured a minimum

Figure 2: E� ect of Inter-stage Precipitation on sulphate on OWRP

Mine Feed Stage 1 RO Feed Stage 1 RO Reject Stage 2 RO Feed Stage 2 RO Reject Stage 3 RO Feed Brine Potable Water

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Figure 3: Scaling Potential at the various plants

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four times daily to ensure the scaling potential of the water is known at all time. Understand-ing the practical limitations of the antiscalant in terms of the water quality being processed and the RO unit con� guration governs the practical RO recovery that can be managed in these complex scaling waters. � e scaling po-tential is reduced by the precipitation reactors on each stage. Figure 3 shows the scaling po-tential as measured by the calcium sulphate saturations.

� e horizontal lines indicate the practical maximum RO recovery that can be operated until scaling begins to occur. Should these boundaries be exceeded, rapid irreversible and severe scaling can results. However, the closer one operates to the maximum RO re-covery, the greater instantaneous production that one can achieve from a given plant. It is apparent to see that the greater the standard deviation of the scaling potential, the greater the risk of scaling incidents. � e precipita-tion steps in the process are able to reduce the scaling potential of the reject by between 3-5 times depending on the plant feed water quality. � e impact of poor control of scal-ing potential is on the second bank of RO membranes installed. An increase in the DP without an increase in temperature indicates scaling. Figure 4 below shows the di� eren-tial pressure increase on Bank 2 of one of the Stage 2 RO units at OWRP since the mem-branes were installed and the associated scal-ing potential of the water. � e data has not been normalised. What one can see is that the rise in the DP is faster at the beginning period

of the operation when there is greater vari-ability of saturation data.

Fouling ControlUltra� ltration membranes are utilised as a pre-treatment to the RO membranes. � e selection of these membranes over conven-tional pre-� ltration was due to the ability to process water of a high scaling potential and produce a � ltrate that has very low suspended solids. Typically, the water that is produced from the UF membranes will have an SDI of below 3 and more generally below 2. It has been observed that the SDI will dri� towards the 3 value as the membranes age. In general, a � ve year life can be obtained from the UF membranes in all of the installations. How-ever, this is a function of the feed water being process and it UF fouling potential. However, if one looks at TOC pro� le within the plants, one can see that there in an increase through the plant. � is is shown for OWRP graphi-cally in the � gure below. � is increase can be observed through all the installations but the extent of the increase varies across the plants.

� e implication of the increasing TOC through the plant is an increase in the fouling potential due to accelerated bacterial growth rates (as measured by change in weekly plate counts) and increased risk of organic fouling. What has in fact been observed is that higher fouling rates through Stage 3 are observed at MWRP and OWRP when one compares the cleaning frequency of Stage 1 and Stage 3 RO operation. However, this is not observed at EWRP where the reverse trend is observed.

Figure 4: Demonstration of e� ect of Scaling Potential on Bank 2 DP Increase

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� is implies other factors besides just the levels of TOC are important when it comes to predicting fouling rate. It must be noted that the cleaning frequency is severe across all installations and is higher than what is expected through the industry for brackish water membranes. � e current observed av-erage cleaning frequency of the membranes across each plant is between 4 days and 14 days depending on the process Stage in op-eration and the time of year. � e operational control of fouling on the plants is not elegant and combines a series of o� ine interventions (biocide, manual cleaning) done periodically to manage the situation. Further work is un-derway to better � nd better mitigation mea-sures.

Salt Passage IncreaseDuring the design phase of the plant, the de-fault salt passage increase of the membrane supplier is used to determine the RO vectors within the plant. � is results in an expected salt passage increase of 15% per year. How-ever, the actual salt passage increase is sub-stantially higher. What has been observed is that there is a much higher increase in the salt passage increase in the � rst few years of op-eration a� er which the salt passage increase � attens out. � is can be seen in the � gures be-low. Figure 6 shows the salt passage increase of a Stage 1 RO skid at OWRP from the � rst

three years of operation and a second RO skid being the last three years of operation. For the new skid, the � rst years operation there was the greatest increase in permeate conductiv-ity at 109% with this deterioration decreasing in years 2 and 3 from 89% and 59% respec-tively. � is is compared to that of the skid of the last three years of operation where the last three years of salt passage increase can be seen at 4.27%. If one looks at a Stage 1 RO unit at MWRP (Figure 7) from new, the perme-ate conductivity increase over the � rst three years of operations is still higher than the de-sign at 100% for the � rst year but only 2% in the second year and 23% a� er year three. It remains to be seen if MWRP Stage 1 RO skid will exhibit the same trends as the OWRP skid as the membranes age. However, even with this higher than expected salt passage increase, the product water produced from the RO skids is still well within the speci� ca-tion for SANS241. Showing that sustainable operation can be realised.

Figure 6 (Right) indicates that baring mechanical damage or scaling, there is a high deterioration of the permeate quality produced from the RO membranes in the early years of operation until it reaches a high point a� er which it � attens out. What is also interesting to note from the trends in Figure 6 is that the impact of temperature on the permeate quality is more pronounced as the

Figure 5: TOC and DOC from feed to brine at OWRP. Similar trend observed on all installations

Mine Feed ST#1 UF Feed ST#1 RO Feed ST#1 Brine ST #2 UF Feed ST#2 RO Feed ST#2 Brine ST#3 UF Feed ST#3 RO Feed ST#3 Brine

Conc

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TOC DOC




membrane ages. � is can even be seen in the new membrane case, where in the � rst year, the di� erence between the permeate quality in the low temperature is not too dissimilar to the high temperature case but the cycli-cal variance becomes more pronounced in year 2 and year 3. � e cyclical component on the older membranes is apparent. � is is not only observed on the skids presented but on a number of skids of di� erent ages within OWRP and EWRP but is not obviously seen at MWRP. � e increased salt passage vari-ability observed is due to the higher basic salt passage coe� cient of the aged membrane, on which temperature acts as a multiplier.

Operations � e control of the upstream pre-treatment and the RO membranes requires careful man-agement. Loss of control of any aspect of the plant will have a direct impact on the qual-ity and quantity that can be produced from the plant. As a result, utilisation of this type of plant for the augmentation of water supply to the community will require a highly skilled team. In addition, continuous monitoring of the quality of the plant as well as the various process units require sophisticated control narratives and automation. Lastly, the opera-tions team will need to be well versed in the operation of membranes speci� cally in terms

Figure 6: Permeate Conductivity increase of New Membranes (le� ) and Old Membranes (right) at OWRP

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Figure 7: Permeate conductivity increase of New Membranes at MWRP of New Membranes - Stage 1 RO Skid

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of when it is required to take o� ine to clean as well as what cleaning recipes to use. � ese recipes � uctuate as the risk of both biological and inorganic foulants within the membrane are both present within these installations.

Conclusion It has been shown that RO membranes can be used to produce a product water that com-plies with the required legislative parameters. � is can be seen by the sustainable operation of three plants, each running for more than three years. � e key to producing acceptable quality water is the use of RO membranes to desalinate the feed water. Sequential precipi-tation minimises scaling potential and energy costs associated with operations of the mem-branes and allows for high plant recoveries. Management of the scaling potential of the feed water with respect to Calcium Sulphate precipitation is paramount for sustained RO operation, failure of which will cause either a loss of membrane area or a loss of produc-tion. In addition to scaling, the levels of TOC increase substantially through the plants. � is increases the risk of organic and biological fouling of the RO membranes. Salt passage increases of the ROs on these plants exceed the initial design expectation by a substantial

factor. However, it is viewed that the initial sharp decline in permeate quality stabilises as the membranes age but there is an increased sensitivity of the permeate conductivity to temperature. � e product water quality is still within the tolerances allowed for drink-ing water despite these challenges. � e plants are operationally complex and require highly skilled sta� to manage the various aspects of the membrane operation. � e HiPRO™ plants that have been in operation for a number of years and all comply with the SANS241 speci-� cations. Two of which are currently servic-ing the community for potable water showing that mine impacted water can sustainably be used to augment existing drinking water sup-plies.

References Hutton B, Kahan I, Naidu T, Gunther P, (2009)

Operating and Maintenance experience at the eMalahleni Water Reclamation Plant. IMWA 2009.

Coleman T.J., Usher B, Vermeulen D, Lorentz S, Scholtz N. Prediction of how different man-agement options will affect drainage water quality and quantity in the Mpumalanga coal mines up to 2080. WRC Research Report No. 1628/1/11 , 2011


producing high quality drinking water from mine impacted ... · standards for drinking water...

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