Water Journal : Water Journal September 2012-1
refereed paper potable reuse water SEPTEMBER 2012 61 by the inverse of RO recovery (e.g., a decrease of 15% from the water diverted); this can have a substantial impact on environmental flows as well as downstream users. This analysis suggests that air, water and land environmental impacts, as well as water availability, need to be seriously considered prior to opting for an RO IDWR option. Finished Water Quality Including Emerging Contaminants Tables 3 and 4 compare finished water quality for total organic carbon (TOC), nutrients, pathogens, total dissolved solids (TDS) and chemicals of emerging concern (CECs). Table 3 includes data from two full-scale plants and Table 4 includes data from six full-scale plants. Although data from identical parameters was not universally available, significant quantities of data allowed for a fair comparison. Examination of these tables reveals the following: • The TOC concentration for the RO-based plant is significantly lower than the GAC-based plant (0.6mg/L versus 2.7mg/L). The TOC for the GAC-based plant is based on a regeneration frequency of 12.5% GAC media regeneration per year. Lower TOC in the GAC-based plant could be achieved with higher GAC regeneration frequencies. • The total dissolved solids (TDS) concentration for the RO-based plant is significantly lower than the GAC- based plant (130mg/L versus 398mg/L) because of RO's ability to reject salts. The TDS concentration from both plants is below USEPA's secondary maximum contaminant level (MCL) for drinking water of 500mg/L. Also note that RO generates a highly saline concentrate stream that can create significant disposal challenges for inland locations. • The total nitrogen concentration for the RO- based plant is significantly lower than the GAC-based plant (0.6mg/L versus a range of 3--16mg/L, which averages 12mg/L). However, the GAC-based plant's product water total nitrogen is mostly comprised of nitrate. Nitrate is intentionally discharged in the 12--16mg/ L range during periods of reservoir stratification to improve reservoir water quality and limited to 3--8mg/L when the downstream reservoir is not thermally stratified. The GAC plant is fully capable of controlling nitrate levels well below the USEPA drinking water MCL of 10mg/L. • The finished water turbidity from both plants is very low. The turbidity of the RO-based approach is higher because of inerts associated with lime addition in the finished water stabilisation approach. • Total coliform and E. coli are not detectable in the finished water for the GAC-based approach. These parameters were not measured at the RO plant, but are expected to also be below detection limits. • In general, CECs at the RO-based and GAC-based treatment plants are very low and in most cases below the detection limits. Some CECs are present in the finished water at both the RO- and GAC-based plants. Conclusions Analysis using a detailed parametric cost model and USEPA-based carbon emissions data indicates significantly higher costs and greenhouse gas (GHG) production for the RO-based treatment approach when compared to the GAC- based treatment approach for plants of equivalent capacity. The capital and operating costs for the RO-based approach are 50% and 350% higher, respectively, than the GAC-based approach. Furthermore, the RO-based approach produces more than twice the GHG emissions. Table 3. Bulk water quality in finished water from indirect potable reuse (IPR) plants (Angelotti, 2010). Parameter Average Finished Water Quality Notes GAC1 RO3 TOC (mg/L) 2.7 0.6 TDS (mg/L) 398 130 Phosphorus (mg/L) 0.06 0.02 Nitrogen (mg/L) 3--16 0.6 GAC1 nitrogen is comprised mostly of nitrate, which is naturally removed in the receiving reservoir; in addition, elevated nitrate is purposefully discharged at times to the receiving reservoir to prevent water quality problems at the downstream drinking water plant by creating anoxic conditions that delay or retard anaerobiosis in the hypolimnion. Turbidity (ntu) 0.2 0.6 RO3 turbidity is elevated because of inerts associated with finished water lime addition. Total Coliform (cfu/100mL) ND NM E. coli (cfu/100 mL) ND NM Notes: ND = not detected; NM = not measured Figure 5. O&M-related CO2 emission estimates.
Water Journal November 2012-1
Water Journal August 2012