Management of BWRO systems using long-term monitoring of feed water quality to avoid future membrane process failure

Figures & data

Fig. 1. Map showing the location of the Bonita Springs Utilities BWRO facility and the wellfield and wells that are used to obtain feed water.

Table 1 Well numbers with construction details

Well no.	Total depth (m)	Casing depth (m)	Casing diameter (cm)
25	317.0	248.7	35.56
26	324.0	254.5	35.56
27	329.2	248.4	35.56
28	308.5	245.4	35.56
29	324.0	256.3	35.56
30	341.4	274.3	35.56
31	257.0	201.2	35.56
32	213.7	198.1	30.48

Fig. 2. Hydrogeologic cross-section of the Bonita Springs Utilities wellfield.

Table 2 Dissolved chloride and TDS concentrations in the water from the production wells at the beginning of production, at the end of 2014, and projections to 40 years

Well no.	Starting dissolved chlorides (mg/L)	Starting TDS (mg/L)	End of 2014, dissolved chlorides (mg/L)	End of 2014, TDS (mg/L)	5-year projection, dissolved chlorides (mg/L)	5-year projection, TDS (mg/L)
25	1,435	2,609	1,620	2,945	1,723	3,137
26	1,769	3,216	1,760	3,200	1,844	3,352
27	1,860	3,382	1,810	3,290	1,745	3,173
28	1,042	1,895	1,200	2,182	1,318	2,396
29	1,298	2,360	1,740	3,164	2,081	3,783
30	1,368	2,487	1,850	3,364	2,092	3,804
31	1,101	2,002	1,280	2,327	1,388	2,524
32	958	1,742	1,300	2,364	1,354	2,462
Average	1,354	2,461	1,570	2,855	1,693	3,078
Well no.	10-year projection, dissolved chlorides (mg/L)	10-year projection, TDS (mg/L)	20-year projection, dissolved chlorides (mg/L)	20-year projection, TDS (mg/L)	40-year projection, dissolved chlorides (mg/L)	40-year projection. TDS (mg/L)
25	1,785	3,245	1,908	3,469	2,155	3,918
26	1,880	3,418	1,952	3,549	2,097	3,813
27	1,737	3,158	1,722	3,131	1,691	3,075
28	1,423	2,587	1,632	2,967	2,051	3,729
29	2,286	4,156	2,705	4,918	3,538	6,433
30	2,363	4,296	2,904	5,280	3,987	7,249
31	1,450	2,636	1,574	2,862	1,823	3,315
32	1,400	2,545	1,492	2,713	1,675	3,045
Average	1,791	3,256	1,986	3,611	2,383	4,333

Fig. 3. Water quality variation with depth at the injection well test site.

Fig. 4. Diagram showing the variation in pumpage and associated dissolved chloride and TDS concentrations in time for the ten-year operating period, 2014–2014 (a) well 25 (b) well 26 (c) well 27 (d) well 28 (e) well 29 (f) well 30 (g) well 31 (h) well 32.

Fig. 5. Plot of average pumping rate per month vs. percentage change in dissolved chloride change.

Fig. 6. Conceptual model used to assess projected future water quality changes within the production aquifer.

Table 3 Bonita Springs BWRO plant specifications and characteristics

Item	Units
1. Product water maximum production capacity (current)	25,000 m^3/d/ 6.6 MGD
2. Projected maximum product water production capacity (20 years)	50,000 m^3/d/13.2 MGD
3. Current maximum required feed water production from wells	30,700 m^3/d/ 8.1 MGD
4. Future maximum feed water production from wells (20 years)	61,400 m^3/d/16.2 MGD
5. Current membrane operating pressure	1,282 kPa/186 psi
6. Number of membrane elements in plant	1,024
7. Number of trains	4(5,680 m^3/d)/4 (1.5 MGD each)
8. Number of high pressure pumps	4
9. Maximum operating pressure of RO feed pumps	3,323 kPa/428 psi
10. Maximum TDS of feed water that can be treated with current BWRO design	8,129 mg/L
11. Bypass feed (10%)	2,272 m^3/d/0.6 MGD
12. Current number of production wells	8
13. Maximum number of wells required at 20 year projected use	16

Fig. 7. Treatment process diagram for the Bonita Springs Utilities BWRO treatment plant.