WATER QUALITY

When the need for groundwater increases, many try to combine water from different sources, which can lead to precipitation and taste changes. EnviDan has developed a concept with detailed analysis.

EnviDan has developed a concept of detailed analysis of the different wells to find the right strategy to mix water of different qualities. This is then combined with targeted treatment for each well.

In EnviDan's concept, primarily a detailed analysis is carried out of the groundwater quality at the different wells. Extensive water chemistry calculations identify combinations of source sites that can produce acceptable drinking water that does not cause precipitation or changes in taste when mixed. A strategy will be developed for mixing groundwater of different qualities.

Different treatment methods are then assessed for specific parameters and source sites are ranked according to their potential for treatment. This takes into account both the water quality of the specific well and the production capacity. Appropriate treatment is proposed for the use of the respective groundwater flow, resulting in increased capacity and security of supply at the waterworks, without the need to treat the entire volume of groundwater.

CASE STUDY

 

The water works has experienced challenges. The best-performing source sites are saturated with calcium carbonate, leading to limescale deposits on the line grid, while others have elevated levels of pesticides or nitrate. One borehole contains manganese around the limit value and three wells show the presence of natural organic matter (NOM). The treatment process consists of UV disinfection and, due to low UV transparency, only a limited amount of water from the NOM affected wells can be used.

Prior to the project, only a combination of two wells (out of six) could be used, which together produced drinking water with an appropriate level of water quality. This significantly limited water production in a situation where capacity needed to be increased and, in order to meet the expected water demand, more source sites would have to be connected.


Table 1. General water quality in the available wells. CaCO3 stability calculated for 10°C with Aqion PRO.

Wella A B/C D E/F
pH 7,5 7,8 (B)
8,2 (C)
7,7 7,8 (E)
8,0 (F)
Hardness Relatively hard Relatively hard Relatively hard E: Very hard
F: Hard
Nitrate, NO3 <10 mg/l, decreasing  -  Approximately 20 mg/l
,=max. recommendation
Approximately 20 mg/l,
=max. recommendation
CaCO3 balance Aggressive,
10 mg/l dissolves
Aggressive
B: 5 mg/l dissolved
C: 0.2 mg/l dissolved
Aggressive,
3 mg/l dissolves
Supersaturated
E: 22 mg/l precipitation
F: 4 mg/l precipitation
Pesticides <<limit value of 0.1 µg/l  -  Close to the limit value    - 
UV-transmission Varying,
77 - 90 %/cm
Low,
B: 80%/cm,
C: 73%/cm
High,
> 90%/cm
Very high,
> 95%/cm

 

OPTIONS FOR TREATMENT

In order to increase capacity, EnviDan has analysed the raw water quality of the wells with regard to the above parameters. Extensive water chemistry calculations have been carried out, including calculations of the calcium precipitation potential (CCPP) for different water combinations. Appropriate treatment methods for specific parameters have been investigated and evaluated. The wells are ranked according to which can best be supplemented with treatment steps, in terms of both the well's specific problems and its production capacity.

The alternatives that have been investigated are the following and the results can be seen in Table 2, below. Depending on the needs, the customer can then choose the appropriate strategy to proceed.

  1. Current system. A and F wells in production, D diverted to a stream to protect the quarry.
  2. Nanofiltration (NF) on partial flow from E/F. Main objective: softening.
  3. Nanofiltration (NF) on the whole flow from B/C. Objective: NOM and manganese separation.
  4. Activated carbon (GAC) filter for D. Objective: to remove pesticides.

Table 2. Combination of wells with and without water treatment. Water quality and treatment capacity parameters.

 Alt  Combination of wells   CCPP10°C
CCPP90°C
UV-trans-missions Capacity Comments
0 A: 30%
F: 70%
1 mg 
22 mg 
94 %/cm  3200 m3/d Status quo, limited capacity, only 2 wells available for optimal quality.
1a  E/F: NF, 35%
B/C: bypass, 65%
2 mg 
30 mg
>97 %/cm 3600 m3/d NF stream 65 m3/h, 13 m3/h reject.
3 wells used, small NF plant.
1b A: 24%
E/F: NF, 16%
B/C: bypass, 60% 
1 mg 
29 mg
>90 %/cm 4900 m3/d NF flow 41 m3/h, 8 m3/h reject.
3 wells used, small NF plant.
2 A, B, C
E, F: changing %
NF for B & C 
Depends on mixture   >90 %/cm >7700 m3/d NF flow max 285 m3/h, max 57 m3/h reject. 5 wells available for optimal quality.
Mixing with appropriate CCPP is possible.
3 A, D
E, F; changing %
GAC filter for D
Depends on mixture   >88 %/cm 7350 m3/d GAC filter max 50 m3/h.
4 wells available for optimal quality.
Mixing with suitable CCPP is possible.

 

PILOT PROJECT

It was decided to proceed with a pilot project for nanofiltration to remove lime from wells E and F. By soaking the water from these wells, plus well A, future water needs could be met. This alternative would also reduce the nitrate content of the water stream being treated. However, the lack of a filter step and the presence of iron and manganese were found to clog the filter regularly and cause operational problems. A solution with softening via ion exchange was, therefore, chosen instead.

In parallel with nanofiltration, a pilot activated carbon test was started for well D. The well has a good production capacity, but due to the presence of pesticides and degradation products of BAM, water is only pumped to a nearby stream. Carbon filters will be installed during the summer of 2021.

Through the detailed analysis of the source sites, an appropriate schedule for mixing water of different qualities could be prepared. It also provided the basis for developing proposals for appropriate treatment of the different volumes of groundwater, which could then be tested. Thanks to the targeted treatment, the treatment methods could be tailored to the respective groundwater flow and several of the wells were put into operation, thus ensuring the water supply to the area.

Elin Hermansson

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Elin Hermansson

0705080760