mpungu vlei report

7/27/2019 Mpungu Vlei Report

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MPUNGUVLEI PROPOSAL

Namibia Water Corporation

Private Bag 13389

Windhoek

Namibia

Report by: Water Quality Services

March 2012


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Water Treatment Services

PROPOSAL FOR NEW WATER TREATMENT

PLANT FOR MPUNGUVLEITable of Contents

1. INTRODUCTION ............................................................................................................................. 3

1.1 Background Information: .......................................................................................................... 3

1.2 Objective: .................................................................................................................................... 3

2. PROPOSED TREATMENT OPTION(s) ....................................................................................... 3

3. PROPOSED TREATMENT PLANT(S)......................................................................................... 5

3.1 Borehole Blending (Only): .................................................................................................... 5

3.2 DT-RO Treatment with Raw Water Blending: ................................................................... 7

4. PROPOSED PILOT WORK ........................................................................................................... 9

4.1 Requirements: ............................................................................................................................ 9

5. WASTE DISPOSAL:........................................................................................................................ 9

5.1 Design of Evaporation Ponds .................................................................................................. 9

5.2 Cost of Civil Works for Evaporation Ponds.......................................................................... 10

6. COST ESTIMATION ..................................................................................................................... 10

7. REFERENCES............................................................................................................................... 11

APPENDIX: ......................................................................................................................................... 12


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PLANT FOR MPUNGUVLEI1. INTRODUCTION

1.1 Background Information:

The Mpunguvlei is a township in the Kavango area. The township is supplied with water from a

number of boreholes in the vicinity. Two specific boreholes (WW22188 and WW30956) provide Class

D (unacceptable) water, with high fluoride and sodium levels. These two boreholes of concern bothhave a maximum abstraction rate of 4 m3/hr. The current peak drinking water demand of the town is

7.8 m3/hr.

The total drinking water demand of the township over a period of 17 years would be 10.8 m 3/hr, this

includes the two boreholes of concern (each 4 m3/hr) as well as the borehole that supplies water to

the school (WW100121) that supplies water at 3 m 3/hr. The fluoride concentration of the latter

borehole (WW100121) is well within the acceptable standards for drinking water and would therefore

not be included in the treatment option determination. It will remain as is.

A new borehole provided by the Ministry of Agriculture, which has a capacity of 8 m3/hr. This borehole

has fluoride levels well below the standards. The standards require fluoride levels in drinking water

not to exceed 1.5 mg/l.

The water quality at the different boreholes in terms of fluoride content can be seen in Table 1 below.

Table 1: Borehole Water Quality and Abstraction Rates

In addition to the high fluoride levels, the sodium for the two boreholes of concern is 546 mg/l and 433

mg/l on average for boreholes WW22188 and WW30956 respectively. This is significantly higher than

the acceptable standard which requires drinking water to be under 300 mg/l.

The new borehole (8 m3/hr) is a Class A borehole with a sodium level of 107 mg/l.

1.2 Objective:

The aim of this document is to propose and cost a treatment option that would bring the fluoride and

sodium levels to within the acceptable standards used at Namwater.

2. PROPOSED TREATMENT OPTION(s)

Reverse Osmosis (Disc Tube RO Module discussed below) and Ion Exchange are typical treatment

options for removal of fluorides and sodium. The core characteristics of the two treatment options with

regards to treating water at Mpunguvlei which has a relatively low TDS content is displayed in Table 2

below. Another option would be to blend existing substandard boreholes with available good quality

boreholes. Such a treatment option would ultimately be the most cost effective choice to reduce

fluoride and sodium levels. All options will nonetheless be explored in this paper.

Boreholes

Numbers

Recommended

abstraction rate

(m3/h)

Fluoride quality

in 2011

Fluoride quality

in 2006

WW100122 8 Not tested 0.7

WW100121 3 0.8 0.7

WW30956 4 4.3 3

WW22188B 4 5.1 6.1


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PLANT FOR MPUNGUVLEITable 2: Ion Exchange versus Reverse Osmosis

1 12 2

3 3

4 4

5 5

6 6

7 78 8

Ease of Operations

Ease of Maintenance

Anion removal (F-

)Cation removal (Na

+)

Strong Acid Regeneration

Fe-free Aluminium Sulphate Regeneration

Waste Disposal

Ion Exchange DT-RO Module

Citric Acid CleaningBrine/Waste Disposal

Constant Product Water Quality

nonselectiveness

Not Labour IntensiveEase of DesignComplex Design

Varying product water quality

Labour Intensive

It is clear from Table 2 that the DT-RO Module seems to be the preferred option for treating water at

Mpunguvlei. It should be noted that a cost comparison could not readily be performed. It is however

assumed that the two treatment options would be in the same order of magnitude regarding costs.

Further preferences for the DT-RO Module are elaborated below.

In 2010 and 2011 the Water Quality Services Division in Namwater conducted two distinct Reverse

Osmosis Pilot Operations at the Epukiro Pos 3 settlement in the east of Namibia and at the town of

Bethani in the south of Namibia. The pilot rig at Epukiro was used primarily to remove nitrates and

hardness. It did however prove to be equally effective in reducing all the other substandard

parameters as well.

The Epukiro pilot rig was initially shipped to Namibia from Senegal where a PhD candidate was

performing similar tests, with the aim of removing high levels of fluorides in that water. He found that

by using the DT-RO Module, at a recovery of 66%, one can obtain fluoride rejections in access of

98% for waters containing fluorides of up to 20 mg/l. He further found that by increasing the recovery

to 80 % the fluoride rejection dropped to 95 %. The same candidate provided the training at Epukiro

Pos 3.

In February 2012, Namwater managed to acquire ownership of the DT-RO Module pilot rig, presently

still in Epukiro Pos 3. Given the small footprint and therefore ease of transportation of the pilot rigtogether with the historical information regarding its performance, it is therefore proposed that the pilot

rig in Epukiro be used at the Mpunguvlei water treatment scheme to test the viability of implementing

a similar plant there. In addition to the above, the Mpunguvlei water has a much lower TDS value than

the waters at Epukiro and at the scheme in Senegal and should therefore provide much better

recovery and much less fouling and/or scaling of the membranes. As a consequence, the membrane

life could be expected to be substantially prolonged.


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PLANT FOR MPUNGUVLEI3. PROPOSED TREATMENT PLANT(S)

3.1 Borehole Blending (Only):

This treatment method is simply a method of mixing substandard borehole water(s) with good

quality borehole water(s) into a final blend stream that is of a better quality than the borehole(s)

with substandard quality as well as within the quality required by water quality standards. In the

Mpungu Vlei case, boreholes WW22188B (4 m3/hr) and WW30956 (4 m

3/hr) is of substandard

quality regarding its fluoride and sodium content. The new borehole WW100122 (8 m3/hr) is of a

good quality.

The borehole blending treatment option includes three different scenarios or alternatives (that is

discussed below) to ultimately reduce the fluoride and sodium concentrations to within the newly

proposed Namibian Water Quality Standards, which is 1.5 mg/l for fluoride and 300 mg/l for sodium.

Currently, the two 4 m3/hr substandard boreholes are being pumped into an elevated reservoir (or

tank). This reservoir will be referred to as the existing mixing tank for the two boreholes. Water will

be extracted from this mixing tank and further mixed with the 8 m3/hr good quality (0.7 mg/l)

borehole water, to produce a blend stream that will satisfy the peak demand as well as the pre-set

water quality required. A pre-set water quality of 1.3 mg/l fluoride is chosen as the driving force

behind all the calculations, since fluoride is the limiting parameter i.e the sodium would

consequently be reduced to within the required standards if the fluoride is reduced to the pre-set

water quality. This pre-set water quality also represents a value that is operationally safe with

respect to the 1.5 mg/l required by the water quality standards.

NOTE: The peak water demand forecast (Table 4) shows that the current peak demand is 7.8 m3/hr

and the peak demand after 17 years would increase to 10.8 m3/hr.

The first blending scenario (Figure 2) shows a blend product stream equal to the current peak

demand of 7.8 m3/hr. This requires an extraction of 1.17 m

3/hr from the mixing tank and a 6.63

m3/hr flow from the new borehole.

4 m3/hr

WW22188B 5. 1 mg F/l

8 m3/hr

4.7 mg F/l

4 m3/hr

WW30956 4. 3 mg F/l

1.17 m3/hr Required Year2010/11

7.8 m3/hr

1.3 mg F/l

6.63 m3/hr Required

(NEW) 8 m3/hr

WW100122 0. 7 mg F/l

20 hr Peak Demand

Forecasted2

Mixing

Tank

(Existing)

1

3

Figure 1: Blending Scenario 1

As a second scenario with the same set-up as scenario 1, the system will only be able to satisfy the

future peak demand for the next 7-8 years (i.e up until 2019/20, at 9.4 m3/hr, from Table 4) if the

new borehole is stretched so that it could run at its maximum capacity of 7.99 m

3

/hr. This new


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PLANT FOR MPUNGUVLEIborehole would therefore run at maximum capacity for over a year (i.e around the years

2018/2019).

4 m3/hr

WW22188B 5. 1 mg F/l

8 m3/hr

4.7 mg F/l

4 m3/hr

WW30956 4. 3 mg F/l

1.41 m3/hr Required Year 2019/20

9.4 m3/hr

1.3 mg F/l

7.99 m3/hr Required

(NEW) 8 m3/hr

WW100122 0. 7 mg F/l

Forecasted

20 hr Peak Demand

2

Mixing

Tank

1

3


For scenario 3 (Figure 4), if we abandon the safe pre-set fluoride product concentration value of 1.3

mg/l for the 1.5 mg/l fluoride concentration required by the standards, blend water can be produced

up until the year 2023/2024 (from Table 4), which is 11-12 years from today. However, this would

mean that we would be producing water at exactly the required fluoride concentration. This set-up

runs the risk that should the plant experience concentration fluctuations, the fluoride concentration

could increase to above the required standards.

4 m3/hr

WW22188B 5.1 mg F/l

8 m3/hr

4.7 mg F/l

4 m3/hr

WW30956 4.3 mg F/l

2 m3/hr Required Year 2023/24

10 m3/hr

1.5 mg F/l

8 m3/hr Required

(NEW) 8 m3/hr

WW100122 0.7 mg F/l

Forecasted

20 hr Peak Demand

2

Mixing

Tank

1

3


Taking the above scenarios into consideration, it is therefore recommended that should the blending

treatment option be implemented, scenario 2 should initially be used followed by scenario 3. This

would mean that for the first 7 years the system would be running on safe mode (1.3 mg/l fluoride in

blend stream) and switch to the borderline (1.5 mg/l Fluoride) system for the next 4 years up until

the year 2023/24 (from Table 4).

The above recommendation would require an additional reservoir that could catch the blended

product water. This would allow the system to be set at a set 1.4 m3/hr from the mixing tank and a

full capacity operation of the new borehole, over the course of the initial 7 years. Thereafter, onlythe mixing tank extraction should be increased to 2 m

3/hr.


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PLANT FOR MPUNGUVLEI

3.2 DT-RO Treatment with Raw Water Blending:

A borehole feed flow rate of 8 m3/hr to a proposed plant will be the driving force behind the

calculations to ultimately determine what the product water quality and flow rates will be when the DT-

RO system is used at Mpunguvlei. The PFD for the proposed plant, including flow rates and water

quality with regards to fluorides and sodium is shown below in Figure 1.

Figure 4: PFD of Proposed DT-RO Plant at Mpunguvlei

A maximum set recovery of 75 % would be used because of the low fouling/scaling tendency of the

Mpunguvlei water. Given the rejection results for sodium and fluoride obtained in Senegal and

Epukiro, the sodium rejection would be fixed at 93% and the fluoride rejection would be fixed at 96%.

The sodium rejection rates at Epukiro and the fluoride rejection rates in Senegal were therefore used

to forecast the rejection rates at the proposed plant. It is assumed that the lower TDS value of the

Mpunguvlei water in comparison to the Epukiro and Senegalese waters will not affect the rejection

rates.


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PLANT FOR MPUNGUVLEITable 3: Relationship between Feedwater salinity, Brine salinity and Recovery

% Recovery 83.29 80 75 70 65.88 65 60 55 50.59 50 45 40 35 30

Concentration Factor 5.98 5.00 4.00 3.33 2.93 2.86 2.50 2.22 2.02 2.00 1.82 1.67 1.54 1.43

Feed TDS = 10,000 mg/l 59829 50000 40000 33333 29308 28571 25000 22222 20237 20000 18182 16667 15385 14286Feed TDS = 9,000 mg/l 53846 45000 36000 30000 26377 25714 22500 20000 18213 18000 16364 15000 13846 12857

Feed TDS = 8,000 mg/l 47863 40000 32000 26667 23446 22857 20000 17778 16190 16000 14545 13333 12308 11429

Feed TDS = 7,000 mg/l 41880 35000 28000 23333 20516 20000 17500 15556 14166 14000 12727 11667 10769 10000

Feed TDS = 6,000 mg/l 35897 30000 24000 20000 17585 17143 15000 13333 12142 12000 10909 10000 9231 8571

Feed TDS = 5,000 mg/l 29914 25000 20000 16667 14654 14286 12500 11111 10119 10000 9091 8333 7692 7143

Feed TDS = 4,139 mg/l 24763 20695 16556 13797 12131 11826 10348 9198 8376 8278 7525 6898 6368 5913

Feed TDS = 4,000 mg/l 23931 20000 16000 13333 11723 11429 10000 8889 8095 8000 7273 6667 6154 5714

Feed TDS = 3,000 mg/l 17949 15000 12000 10000 8792 8571 7500 6667 6071 6000 5455 5000 4615 4286

Feed TDS = 2,858 mg/l 17099 14290 11432 9527 8376 8166 7145 6351 5784 5716 5196 4763 4397 4083

Feed TDS = 2,000 mg/l 11966 10000 8000 6667 5862 5714 5000 4444 4047 4000 3636 3333 3077 2857

Feed TDS = 1400 mg/l 8376 7000 5600 4667 4103 4000 3500 3111 2833 2800 2545 2333 2154 2000

Feed TDS = 1,000 mg/l 5983 5000 4000 3333 2931 2857 2500 2222 2024 2000 1818 1667 1538 1429

Feed TDS = 500 mg/l 2991 2500 2000 1667 1465 1429 1250 1111 1012 1000 909 833 769 714

NOTE: The shaded figures ( 15000 mg/l) will cause rapid fouling/scaling in the DT-RO Module

Brine TDS(mg/l) at different Recoveries

Table 3 (courtesy of El-Manharawy and Hafez, 2001) illustrates the effects of feed water TDS, for

given recovery rates, on corresponding brine TDS values. The accuracy of the table is confirmed in

the DT-RO Pilot Plant Operations at Epukiro Pos 3: Namibia Sub-standard Water Quality

Improvements report.

The table shows that for a feed TDS of 1400 mg/l, a brine TDS of approximately 8376 mg/l can be

expected at a recovery of 80 %, which would be well below the rapid fouling/scaling region for a

reverse osmosis system ( 15 000 mg/l brine TDS). It is however proposed to run the DT-RO Module

at a safer recovery of 75 % with the brine TDS at 5600 mg/l at a feed TDS of 1400 mg/l. This would

allow the system to be used over a much longer period i.e. prolong the membrane life. It will also

mean that should a higher recovery be needed the plant recovery could be increased to 80% while

not compromising the integrity of the system.

These proposed plant conditions and performances will yield a production rate of 6.4 m3/hr. The peak

demand table below (courtesy of Water Supply) shows that the proposed plant will be able to produce

enough acceptable drinking water up until 2019 (for a total of 7 years). The product water from the

plant will be added to the existing 3 m3/hr borehole to be able to establish the 7 year running period of

the proposed plant.


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PLANT FOR MPUNGUVLEITable 4: Forecasted Drinking Water Peak Demand of the Mpunguvlei Township over 17 years

4. PROPOSED PILOT WORK

It is anticipated that the Pilot Rig currently in Epukiro be transported to Mpunguvlei to conduct a pilot

test on the water, to confirm the viability of using such a plant at the Mpunguvlei water scheme.

4.1 Requirements:

1. Transportation of DT-RO Pilot Rig to Mpunguvlei

2. Chemicals that will be used for testing, must be made available

3. Repair leakages of the Pilot Rig

4. Perform an overall maintenance of the Pilot Rig

5. Electricity must be made available for the DT-RO Module

6. Module must be housed under roof

7. Operator training must be done

8. Holding tanks as well as piping must be made available

9. Brine disposal options should be considered

5. WASTE DISPOSAL:

5.1 Design of Evaporation Ponds

The proposed plant at Mpunguvlei will produce a brine flow rate of 1.7 m3/hr. This brine will be routed

to two evaporation ponds situated appropriately close to the plant. The two ponds will be used

interchangeably to allow for maximum evaporation during the year. The required area of one of the

ponds can be calculated using the general material balance equation below. The calculations will

include the brine flowing into the ponds, the portion of water evaporated as well as the assumed

annual rain downfall.

Material Balance:

The generation term will equal zero as there will be no generation of material inside the ponds. Theabove generalized equation will yield the following:

Year

nnua

Sales

m3/annu

m

Annual

Production

m3/annum

Average Daily

Production

m3/day

verage ay n

Peak Month

Production

m3/day

20hr

peak

daily m3

20010/11 37 067 38 179 104.6 156.9 7.8

2011/12 37 808 38 943 106.7 160.0 8.0

2012/13 38 565 39 721 108.8 163.2 8.2

2013/14 39 336 40 516 111.0 166.5 8.3

2014/15 40 123 41 326 113.2 169.8 8.5

2015/16 40 925 42 153 115.5 173.2 8.7

2016/17 41 743 42 996 117.8 176.7 8.8

2017/18 42 578 43 856 120.2 180.2 9.0

2018/19 43 430 44 733 122.6 183.8 9.2

2019/20 44 298 45 627 125.0 187.5 9.4

2020/21 45 184 46 540 127.5 191.3 9.6

2021/22 46 088 47 471 130.1 195.1 9.8

2022/23 47 010 48 420 132.7 199.0 9.9

2023/24 47 950 49 389 135.3 203.0 10.1

2024/25 48 909 50 376 138.0 207.0 10.4

2025/26 49 887 51 384 140.8 211.2 10.6

2026/27 50 885 52 412 143.6 215.4 10.8


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PLANT FOR MPUNGUVLEI( )

Rearranging,

Assuming a PAN evaporation rate of 2900 mm/year for the Mpunguvlei area and a maximum pond

depth of 2 meters, gives the following pond area calculations:

Since there will be two identical ponds adjacent to each other, their area will both be 3264 m2.

5.2 Cost of Civil Works for Evaporation Ponds

Assumptions:

Cost of Civil Works is N$ 50/m3

Method of construction is landfill

Number of ponds = 2

Distance between ponds = 3 m

Assuming rectangular block type ponds

Volume of earth to be removed:

The pond depth is 2 m and the calculated cross-sectional area is 3264 m 2. This means that a total of

6528 m3 volume of earth should be removed for one pond and 13 056 m 3 volume of earth for both the

ponds. The cost estimate for constructing the two ponds will therefore be in the order of N$ 50/m3 x

13 056 m3 = N$ 652 800.

6. COST ESTIMATION

For the Epukiro Plant Proposal at an overall recovery of 75.57% and a product water flow rate of 5.6

m

3

/hr (drinking water demand), the capital cost was given by Pall Company to be N$ 1.8 million(formal quotation, excluding Evap Ponds). The capacity of the proposed plant at Epukiro is 6.3 m

3/hr

(i.e DT-RO throughput).

The proposed water production for the Mpunguvlei water scheme is 6.5 m 3/hr, with a DT-RO

throughput of 6.5 m3/hr. The Mpunguvlei proposed plant capacity is therefore in the same order-of-

magnitude as that of the proposed plant at Epukiro Pos 3. This means that the cost of the two plants

would be equivalent. The much better quality feed water to the plant at Mpunguvlei will consequently

further reduce the cost of such a plant, but that issue will not be explored in the cost estimation of this

document.


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PLANT FOR MPUNGUVLEIThe proposed plant at Mpunguvlei would hence be in the order of N$ 1.8 million (excluding

Evaporation Ponds). Further operational costs would be negligible due to low chemical usage and

ease of operations of the DT-RO Module.

The total estimated cost of the proposed plant including the evaporation ponds would thus sum to N$

2,452,800.00.

NOTE: This is only a conservative cost estimate.

7. REFERENCES

1. J. A. Beukes et al, DT-RO Pilot Plant Operations at Epukiro Pos 3: Namibia Sub-Standard Water

Quality Improvement, Namwater, 2011

2. C.K.Diawara et al., Performance of Nanofiltration (NF) and Low Pressure Reverse Osmosis

(LPRO) Membranes in the Removal of Fluorine and Salinity from Brackish Drinking Water,

Senegal, Journal of Water Resource and Protection, November 2011, 3, 912-917.


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PLANT FOR MPUNGUVLEIAPPENDIX:

Borehole No Sample Date pH Turb NTU Cond mS/m TDS Calc. Na in mg/l K in mg/l Ca as CaCO3 Mg as CaCO3 SO4 in mg/l NO3 as N in mg/l F in mg/l Cl in mg/l

WW100122 20/02/2006 8.1 19.8 64.2 430.14 107 12 67.5 108.3 23

mpungu vlei report

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