MechChem Africa

Peter Middleton argues the case for seeing all wastewater, including sewage and AMD, as vital water resources for potable and industrial use and for the widespread adoption of high-recovery water reclamation and treatment technologies. 

Peter pic latestMost of us remember learning about the water cycle in primary school, which morphed into the hydrosphere in our secondary years. In terms of recyclability, water is fantastic. But haven’t we been lazily allowing nature to do too much of our water purification work?

For human survival, we need clean (potable) drinking water. For agricultural crops and domestic livestock we have to have fresh water for irrigation and watering purposes, which need not be quite as potable. For our ongoing health, we use water for washing and flushing toilets, while industry consumes water for cooling and processing in a host of different ways. Clean potable water from our purest springs or our most advanced purification plants quickly becomes contaminated, polluted and even poisoned.

Fortunately, as pointed out by Veolia’s Chris Braybrooke in this issue, all wastewater, no matter how contaminated, can be recovered and treated to any level of purity.

Water scarcity, recently in sharp focus across South Africa and still an acute problem in the Western Cape, is now of global concern. Water resources are becoming scarcer and, therefore, the reuse of wastewater, which we have recklessly regarded as a problem to be moved elsewhere, is becoming more and more attractive.

Not only is the water valuable, but also contaminants such as the organic matter, nitrates and phosphates in sewage can be recovered for fertilisers and, for minewater, many of the dissolved metals can be beneficiated.

In a 2016 study focused on the reuse of organic matter and phosphorus from Amsterdam’s wastewater system – Wastewater as a resource: Strategies to recover resources from Amsterdam’s wastewater – authors Van der Hoek, De Fooij and Struker show the water flows in Amsterdam’s system. For 2013, Waternet produced 57.2-million m3 of drinking water for distribution in Amsterdam. Only about 2.5% of the water is ‘lost’, while the remaining 97.5% is combined with storm water and infiltrated ground water and transported via sewers to wastewater treatment plants (WWTPs).

While this paper focuses on the recovery of phosphates by producing struvite (magnesium ammonium phosphate or NH4MgPO), the biggest WWTP of Amsterdam produces: 11 300 Nm3 of biogas; 22.7 MWh of electricity from incinerated solid waste; 55 GJ of direct boiler heating from the residual heat of incineration; along with a total of 74.9-million m3 of treated water, which is returned into the region’s natural surface water resources.

We retain a notion that the water will be purer if the environment has some role.

There is a shining example of wastewater recycling closer to home, however, in Windhoek. The Goreangab Reclamation Plant, originally constructed back in 1968, is one of the few direct potable reuse plants in the world. From Windhoek wastewater, the plant produces 21 000 m3/day of potable water, which is returned directly back into the municipal drinking water network. None of the purified water is discharged into the river systems.

While the costs of such networks are high, in water stressed areas where desalination might be the only other reliable water option, does it not make sense to contain the water for as long as possible in a closed loop system?

In our Innovative feature for this month, Multotec’s Carien Spagnuolo tells of an industrial closed loop water treatment solution being used in the Middle East to maximise water reuse at an antimony roaster. This multi-technology treatment system for the scrubber and cooling tower blowdown water, which is contaminated with toxic antimony and arsenic, embeds all of the elements of an ideal solution for our mine wastewater and acid mine drainage (AMD) water treatment problems.

The first step involves traditional precipitation and clarification – dosing with ferric chloride produces a metal sludge in a settling tank. AMD dosing with lime is widely practised in South Africa for AMD treatment. This neutralises the acidity and removes the dangerous heavy metals, but it leaves the discharge water highly salinic.

In the second step at this treatment plant, the DeSALx® process, which is built around a continuous ion exchange (CIF®) technology is being used to extract the multivalent salt ions – typically SO42- and Ca2+.

This leaves only the monovalent ions such as Na+, K+ and Cl- and some sulphite ions, all of which are highly soluble, for removal by a reverse osmosis plant in the final treatment step. The net result is a water recovery rate greater than 90%, compared to 60 to 70% if only desalinating using reverse osmosis.

Is it not time to start thinking of all wastewater, including sewage and AMD, as valuable water resources? Potable and industrial quality water can be produced using a variety of high recovery technologies and contaminants can be removed for safe discarding or reclamation, leaving our natural river systems healthy and available for agricultural and other uses.

Download the PDF.

BANNER 8

Contact MechChem Africa

Title: Editor
Name: Peter Middleton
Email: mechchemafrica@crown.co.za or peterm@crown.co.za
Phone: +27 11 622 4770
Fax: +27 11 615 6108

Title: Editor
Name: Glynnis Koch
Email: mechchemafrica@crown.co.za
Phone: +27 11 622 4770
Fax: +27 11 615 6108

Title: Advertising Manager
Name: Brenda Karathanasis
Email: brendak@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

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