Recharge systems for protecting and enhancing groundwate

TOPIC 1
Alternative recharge systems / Recharge systems 239
The UF and RO system work at a recovery of respectively 87 and 75%. The concentrate is drained in the nearby
canal together with the part of effluent that has not been treated at ‘Torreele’. To prevent scaling and biofouling on
the RO membranes, scale-inhibitor and monochloramines are dosed to the UF filtrate prior to the RO process.
Quality of infiltration water
Initially the infiltration water was composed of 90 % RO filtrate and 10 % MF filtrate. Re-mineralising RO filtrate
with UF filtrate resulted in a quality that matched the natural dune water (Table 1). During the first two years of
operation, from time to time atrazine, simazine and diuron were detected in the effluent; other pesticides were
never found. If present, the level of atrazine in the effluent was so low (maximum of 0,1 µg/l) that it could never be
detected in the infiltration water. However as they were not removed by UF, simazine and diuron could be found in
the infiltration water. The levels were low, respectively maximum 0,12 and 0,06 µg/l, because comparative tests
showed that they were removed between 97,6 and 98,6% by RO. These results confirmed tests performed during
the pilot tests on removal of estrogenic activity. Based on the response of the recombinant yeast estrogen assay to
the unaltered samples, the estrogenic activity of the effluent was moderate to low and in the filtrates it was lower
compared with the effluent (Van Houtte, 2000). Pauwels (2003) confirmed those results on the full-scale plant,
but no estrogenic activity could now be detected on the filtrates. Recent research showed that the removal of ionic
pharmaceutical residues and pesticides by the RO membranes used at the ‘Torreele’ plant exceeded 95 % (Drewes et
al., 2005).
The IWVA decided in May 2004 to use no longer UF filtrate as part of the infiltration water. The RO filtrate was remineralised
using sodium hydroxide. Since then the infiltration contained less organics and the nutrient content
was lower, which should benefit the infiltration. On one exception for diuron (0,016 µg/l), pesticides were no
longer detected in the infiltration water. As the RO removed all bacteria and viruses, the UV installation could be
stopped.
Table 1. Quality of infiltration water produced at the ‘Torreele’ plant
Infiltration water
(July 2002 - May 2004)
Infiltration water
(May 2004 - February 2005)
Infiltration water
standard
pH
Conductivity (µS/cm)
Chloride (mg Cl/l)
Sulphate (mg SO 4 /l)
Sodium (mg Na/l)
Total hardness (°F)
Nitrate (mg NO 3 /l)
Ammonia (mg NH 4 /l)
Total phosphorous (mg P/l)
Total Organic Carbon
5,70 – 7,67
150 (35 – 262)
22 (2 – 36)
10 (6 – 17)
18 (5 – 30)
3,6 (1,8 – 5,8)
6,9 (1 – 16)
0,31 (< 0,05 – 0,84)
0,1 (< 0,1 – 0,3)
0,9 (0,5 – 2)
6,05 – 7,16
49 (22 – 73)
2,5 (1 – 4)