Comparison of SAR (sodium adsorption ratio) between RO and NF processes for the reclamation of secondary effluent

Abstract

Secondary effluent reclamation and reuse has been considered as an alternative for agricultural irrigation water. Whilst all constituents in the reclaimed wastewater could affect plant growth and soil characteristics, the most important parameters for agricultural irrigation are salinity and SAR (Sodium Adsorption Ratio). Salinity affects the availability of crop water and sodium causes clay soils to disperse. Membrane technologies, especially NF (Nano-Filtration) and RO (Reverse Osmosis), have played in a key role reclaiming the secondary effluent. RO can remove monovalent and divalent cations simultaneously. However NF processes reject preferably divalent cations and most monovalent ions are allowed to pass through the NF membranes. This could make them have different SAR values for both NF and RO processes. Therefore the primary objective of this study is to examine if the SAR values of the reclaimed water could be changed while they undergo NF and RO processes. The measured SAR values of the secondary effluent, NF permeate, and RO permeate were 1.78, 4.67, and 0.72 respectively. The SAR value after NF (4.67) increased to more than twice that of the feed solution, whereas the SAR of the RO permeate decreased to 0.72. In general, the higher SAR the water has, the greater risk the soils have. Although the SAR value after NF was within the safe range, this increased SAR value will affect permeability of soil, thus limiting the reclaimed wastewater use for as agricultural irrigation water. Consequently, when the NF system is used for the reclamation of the secondary effluent, SAR has to be examined first because potentially it tends to increase the SAR value.

Salinity and specific ion effects on onion establishment in relation to disposal of desalting concentrates

“…In the examples listed in Table 1, the Na to (Ca + Mg) ratio of NF concentrate from the Rio Grande return fl ow [5] decreased from 1.47 to 0.65. Likewise, the Cl to SO 4 ratio of the municipal effl uent studied by Chang et al [4] decreased from 2.28 to 0.85. Salinity of irrigation water will inevitably increase upon application of NF concentrates to irrigation water stream.…”

“…Since NF concentrates are enriched with divalent ions, some suggest that its application to irrigation stream would not be deleterious, or can be even benefi cial, as low SAR improves soil structure [15]. However, as discussed in the introduction, the SAR of NF concentrates is not substantially lower than that of the feed water, although it is lower than that of RO concentrates [2,4,5,14,16]. In addition, the formation of sulfate ion pairs with divalent cations further reduces the replacement of exchangeable Na from the cation exchange sites [17].…”

“…Some examples are shown in Table 1. In the municipal effl uent reported by Chang et al [4], and the Rio Grande return fl ow tested by Riley [5], the SAR of the NF concentrate was only slightly lower than that of the feed water, although it was signifi cantly lower than that of RO concentrates. In the situations involving groundwater and the Rio Grande River, the SAR of the concentrate from the NF process was actually greater than that of feed water [2,6].…”

Wastewater reclamation and reuse in China: Opportunities and challenges

“…The results of Oron et al (2008) showed an electro conductivity (EC) removal Table 1 -Cost and benefit of wastewater reuse (Fan et al, 2013 can remove sodium ions and divalent cations, and thus it is very effective for the decline of sodium adsorption ratio (SAR). Chang et al (2005) found a SAR removal of 61% was observed for an initial SAR of 1.8 with the use of RO. For disinfection namely the removal of pathogens, the recommended technology by several guidelines is ultraviolet treatment .…”

Saudi wastewater reuse standards for agricultural irrigation: Riyadh treatment plants effluent compliance

“…Sodium (Na + ), chloride (Cl À ), and boron are the ions of most concern among the specific toxic ions. Salinity directly affects the availability of crop water, while sodium causes clay soil to disperse (Chang et al, 2005). A limit on sodium content is not included in the 2006-MWE standards, while chloride limits are not included in either the previous or the 2006-MWE standards.…”