Guidelines for softening and desalination of industrial wastewater for reuse

BS ISO 23044:2020 pdf free.Guidelines for softening and desalination of industrial wastewater for reuse.
Water quality indicators should include TSS, TOC, COD, pH, temperature, TDS, the species and concentrations of ions.
The product water from wastewater softening and desalination processes is recommended to be reused for urban non-potable water, environmental water, and as pure or ultrapure water for cooling water, boiler feed water, process water, rinse water, and so onll.
The process selection of wastewater softening and desalination processes should be determined after technical and economic comparison based on factors such as influent quality, product quality, quantity requirements, site conditions and environmental protection requirements.
The wastewater needs to be pre-treated if necessary, before being fed into softening and desalination devices.
The selection of pre-treatment process should consider the quality of wastewater, influent quality requirements for softening and desalination processes, water treatment volume and test data. Besides, the operational experience of similar projects should be referred, combined with local conditions. Finally, users can determine which technology to adopt through technical and economic comparison.
Minimizing the discharge quantity of waste acid, waste alkali, waste residue and other harmful substances are important in the selection of softening and desalination processes or device. Measures for treating and disposing these wastes should be taken to meet the relevant environmental protection requirements.
Waste liquid (e.g., regeneration liquid of ion exchange resin process, concentrate of reverse osmosis process, etc.) disposed from the softening and desalination processes should be collected separately according to the characteristics of wastewater quality.
Process flow diagram of industrial saline wastewater treatment for reuse is shown in Eigur1.
5 Requirements for influent quality
Influent quality requirements for softening and desalination device are shown in Table 1. It is noted that data provided in this table is all in advisory typical ranges, which is suggested to be applied according to specific conditions, as well as the manufacturer’s specifications. The parameters listed in Table 1 are also illustrated as follows to show its effect on softening and desalination devices.
a) Silt density index (SDI) reflects the content of particles, colloids, and other objects in influent that can block softening and desalination devices. SDI values higher than the limit can easily block the membrane which will lead to fouling, thereby shortening the operating life of the membrane.
b) Turbidity represents the concentration of undissolved matters in influent that reduce transparency. These undissolved matters can adhere to surface of ion exchange resin, and then block the exchange channel or pollute resin. It can also cause membrane fouling.
c) Water temperature can affection exchange rate and ion absorption ability of resin. It also can affect membrane flux and TDS removal ability of membrane.
d) pH can affect TDS removal ability of membrane and shorten its operating life if exceed typical range.
e) Chemical oxygen demand refers to organic matters which can easily pollute anion exchange resin, because it is difficult to precipitate after the reaction with the anion exchange resin.
f) Appropriate residual chlorine can ensure the sterilization ability for water quality. However, resin is combined with macromolecular organic compounds those can be easily oxidized by high concentration of chloride to break the chemical structure, and then ion exchange ability of resin would be weakened. High residual chlorine can also oxide membrane element and make an irreparable damageL11l.
g) Iron and manganese can be intercepted by resin to form adsorbent that is not easy to wash off. The resin would lose function as the reaction is not reversible. In addition, both iron and manganese can accelerate the oxidation of the membrane and cause irreversible damage to the membrane element.
h) Lower electrical conductivity reflects lower ion content, which is beneficial to form a larger electric potential gradient. More cations and anions would be generated along with the increasing of water dissociation degree. Then the regeneration ability of resin can keep well.
1) High total exchangeable anions can reduce the resistivity of effluent, and then larger running current should be set. However, larger running current can increase the system current and residual chlorine, which is not beneficial to the membrane.
Industrial wastewater should be pre-treated before being fed into the softening and desalination devices to improve water quality by removing particles, TSS, organic matters, etc. Pre-treatment processes consist of conventional treatment and tertiary treatment. Popular pre-treatment processes are shown in Annex A. Combination of those technologies can be adopted according to the quality of wastewater, influent quality requirements of softening and desalination processes, technical feature, cost and so on. Besides, experimental data from lab scale study or similar engineering experience also should be referred.
6 Softening process
Based on hardness and alkalinity of the influent and the water quality requirements of the effluent, softening process can adopt chemical precipitation, ion exchange resin or a combination of the two technologies121. Recommended softening processes are illustrated as follows. Table 2 can also be referred for softening process selectionllI11l.
a) The effects of ion exchange methods are stable and accurate, including single-stage sodium resin, two-stage sodium resin, H-type resin in series with sodium resin, and weak acid cation resin, etc.
b) As to chemical precipitation, the most common precipitants are lime, soda ash and sodium hydroxide.
c) Single-stage sodium resin cannot remove alkalinity; thus, it is suitable for treatment of wastewater with low alkalinity and hardness.
d) Neither can two-stage sodium resin remove alkalinity. The two-stage sodium resin is applicable to treat wastewater with low alkalinity but high hardness.
e) Lime-soda ash softening is an option. Lime softening can remove carbonate alkalinity and soda ash can remove non-carbonate alkalinity.
f) Lime softening combined sodium resin can simultaneously remove hardness and alkalinity, so it is suitable for wastewater with high hardness of carbonate, low excess alkalinity or no excess alkalinity. Lime softening treatment has the advantage of low cost, but there are shortcomings like labour intensity, poor working conditions and that non-carbonate calcium hardness is not affected by treatment with lime softening alone.
g) H-type resin in series with sodium resin is suitable for wastewater with high hardness and high alkalinity.
h) H-type resin in parallel with sodium resin can simultaneously remove hardness and alkalinity, so it is suitable for wastewater with high hardness of carbonate and high alkalinity.
i) Weak acid cation resin in hydrogen form can remove alkalinity but not suitable for the removal of hardness.
j) Nanofiltration membranes can intercept calcium and magnesium ions in water, radically reducing the hardness. However, it has strict requirements for the pressure of influent and high investment and operation cost.BS ISO 23044 pdf free download.Guidelines for softening and desalination of industrial wastewater for reuse