Water salinity is a key issue when discharging treated wastewater to the environment.
It is its TDS content that is an indication of how much dissolved salts are in the water and if too high it poses a risk to the environment. TDS is Total Dissolved Salts. Understanding Industrial Trade Waste: TDS.
High TDS degrades natural river ecosystems and when irrigated it limits crop production in the long-term. High salinity and sodicity in discharge can cause soils to become sodic and can degrade soil structure, making it difficult for water and plant roots to penetrate the soil.
What causes this?
When salts such as sodium chloride, sodium sulphate, calcium sulphate, calcium chloride, magnesium chloride and sodium bicarbonate are in water, they dissociate into positively charged cations and negatively charged anions.
Depending on the soil composition, the cations adsorb to the permanent negative charge of clay. This causes the clays to swell in water and become more dispersive and separate from one another. Dispersed clay causes the permeability of the soil to decline and become almost zero. Reduced permeability of soil leads to the increased likelihood of the soil to remain waterlogged. This reduces aeration of the soil and makes it difficult for penetration of plants roots and plant growth.
What is SAR?
Sodium adsorption ratio (SAR) is a parameter of water to indicate its water quality. It is a measure of the relative amount of dissolved sodium (Na) in water compared to the amounts of dissolved calcium (Ca) and magnesium (Mg) and is defined by the equation:
When the SAR of water is > 3 the water is sodic and can increase the exchangeable sodium percentage (ESP) of the soil. ESP indicates the likely effects that a soil may have with respect to structural stability, because Na+ ions favour dispersion and Ca2+ and Mg2+ favour flocculation.
When the ESP of a soil is > 6, it is likely to adversely affect soil structure and the soil is likely to disperse causing reduced infiltration, percolation and drainage, cloddy seedbeds and poor seedling emergence if the surface becomes crusted and seals.
The adverse impact of sodicity in water is related to its salinity (ECw). EC is the abbreviation of electrical conductivity. The more salt in the water, the higher the ECw. TDS is also calculated based on the electrical conductivity of the water.
Summary guidelines for interpreting SAR values are:
- < 3 no problem as the water is non sodic.
- 3 to 6 medium risk to soil structure and penetration on clayey soils if ECw is < 1.5.
- > 6 has increasing effect on all soils at low to moderate salinity and starts to reduce growth of most crop and pasture plants.
- > 9 severe risk of increasing soil sodicity on most soils.
Occasionally an SAR limit is applied to wastewater discharging to sewer. In this instance the local sewage treatment plant (STP) will be irrigating with their treated water and the council or water authority ensure the suitability of the treated water in terms of sodicity by applying this SAR limit.
Alternatively, a sodium limit is applied instead of an SAR limit. The reasoning behind an SAR or sodium limit is the same.
Similarly, industrial wastewater treatment plants which discharge to the environment are faced with the same challenges, where a strict limit on SAR or Sodium is enforced by the EPA.
The costs for sodium management are significant, as SAR limitations on receiving soils mean irrigators need more land to reduce the soil sodium load (kg Na/m2 or kg Na/Ha) or they are forced to truck water from site.
The sodium mass discharged from site is usually attributed to one or more of three common sources:
The water supply to the factory. Increasing water efficiency reduces the mass of sodium entering the facility within the water supply. This is particularly the case for facilities utilising saline bore water – often reverse osmosis is employed to reduce the salt entering sensitive processing areas.
The use of caustic (sodium hydroxide) solutions within clean-in-place (CIP) or ion exchange (IX) regeneration. CIP and IX regeneration efficiency are both critical to minimising the sodium load sent to trade waste or the environment.
Historically, the use of potassium hydroxide to replace caustic in CIP processes had been a common approach to minimise SAR within wastewater. This is becoming less sustainable, due the impacts of kidney diseases on grazing livestock as well as the higher cost for potassium-based cleaners.
An alternative solution which is gaining significant traction within industry due to the significant environmental and economic benefits it provides is the use of caustic recovery. Hydroflux caustic recovery solutions allow for the reuse of 90-95% of waste caustic solutions, even within sensitive food and beverage applications. This in turn reduces the sodium concentration within wastewater by >90%.
The use of caustic at the wastewater treatment plant for pH correction.
Here the SAR can be reduced by replacing caustic with calcium hydroxide or magnesium hydroxide; this can be seen in the formula above where the Na+ ions are replaced with Ca2+ and Mg2+. The use of these chemicals also improves phosphorus removal and aids in clarification of the wastewater and the performance of wastewater sludge dewatering equipment.
SAR levels and sodium mass loads within wastewater are a significant concern for the Australian environment. Municipal water authorities and industrial water users must drive towards a low sodium economy. Through the implementation of simple, proven technology we can significantly reduce the impact of sodium on precious farmland. These simple solutions will help enable water authorities and industry to interact with the surrounding landscape for generations to come.
Hydroflux can assist with alternative chemicals at the wastewater treatment plant to reduce treated water SAR without impacting on plant performance, and can help optimise RO systems, and offer caustic recovery solutions to allow reuse of up to 95% of the waste caustic solution.
About the Hydroflux Group
The Hydroflux Group comprises eleven companies based in Australia, New Zealand, Fiji and the UK, providing design and build, equipment, processes and operational services in water and wastewater treatment. Hydroflux Utilities provides many forms of water or wastewater plant support programs including, regular service and preventative maintenance contracts, chemical and spares supply and operation contracts for both industrial and municipal customers.
Hydroflux Utilities is a member of the Hydroflux Group.
Hydroflux Utilities NZ Ltd
Level 26, PwC Tower, 188 Quay St
Auckland 1010 New Zealand
P +64 9 352 2052