Diminishing water supplies, population growth, intensification of industry and severe drought are all factors that lead to wastewater re-use as a large part of the answer to the water crisis in South Africa.

Direct & Indirect use                                       

Indirect Potable Reuse

Indirect Potable Reuse (IPR) is the reclamation and treatment of water from wastewater (usually sewage effluent) and the eventual returning of it into the current/natural water cycle well upstream of the drinking water treatment plant.  The point of return could either be into a major water supply reservoir, a stream feeding a reservoir, or into a water supply aquifer where natural processes of filtration and dilution of the water with natural flows aim to reduce any real or perceived risks associated with eventual potable reuse.

The practical distinction with potable reuse relates to temporal and spatial separation between wastewater treatment, the environment and consumption. Existing planned indirect reuse schemes generally incorporate extensive separation to minimise health concerns and public opposition.

IPR can also happen unknowingly when wastewater enters the natural water system (creeks, rivers, lakes, aquifers), and is eventually extracted from the natural system for drinking water being unaware that the natural system contains treated wastewater.

Direct Potable Reuse

Direct Potable Reuse (DPR) can be defined as either the injection of recycled water directly into the potable water supply distribution system downstream of the water treatment plant, or into the raw water supply immediately upstream of the water treatment plant. Injection could either be into a service reservoir or directly into a water pipeline. Therefore, the water used by consumers could be either undiluted or slightly diluted recycled water.  In this definition, the key distinction IPR is that there is no temporal or spatial separation between the recycled water introduction and its distribution to consumers. Public perception of what extent of separation is required for reuse to become indirect dictated the original definition.

Some people are of the opinion that direct re-use should be limited to industrial and agricultural purposes, but with continued water shortages and municipalities’ efforts and expenses in a struggle to upgrade their water treatment facilities to meet the non-stop demand for water, it makes more sense to treat the water even further at the same plant for potable use than to dump the water back into the rivers and extracting it again later downstream at another plant.

Direct re-use greatly reduces operating costs, conserves energy usage and creates a smaller carbon footprint.

Water supplies derived from ground water or surface water like rivers – which normally acts as environmental buffer for treated waste water,  are the primary source of indirect waste water re use.

QFS’s current re-use projects and the reason behind them

Each re-use project is unique as the final use of the water varies. The different re-used projects QFS are delivering in 2016 / 2017 is proof of this statement.

Project 1

Driver:             Not sufficient potable water available – Direct Potable Re-use

Volumes:         3 MLPD

Technologies:  Sand filtration, Ultra-filtration and Reverse Osmosis

Water use:      Potable

Project 2

Driver:             Supplement potable water available – Indirect Potable Re-use

Volumes:         2 MLPD

Technologies:  Sand filtration, Ultra-filtration and Activated Carbon

Water use:      Potable

Project 3

Driver:             Potable water replacement

Volumes:         2 MLPD

Technologies:  Sand filtration, Ultra-filtration and Activated Carbon

Water use:      Irrigation

Technology required for DPR

In the area of waste water re-use, advance technologies are used to provide a multi-barrier approach to achieve the Log removal required for the specified contamination. Typical technologies used are Ultra filtration, Reverse Osmosis, Ozone, biological activated carbon, granular activated carbon and Ultraviolet light.

The main contaminants can be described with the following headings:

  • Suspended solids – Macro filtration in the order of 10 – 40 micron for removal of the bigger particles. Typical gravity sand filters or Disc filters are used for this purpose;
  • Microbiological – Includes Viruses, Bacteria and Protozoa;
  • Inorganic salts – This term includes the whole spectrum of dissolved inorganic solids like Sodium and Calcium;
  • Metals – Most common metal would be Iron;
  • Micro-organics – This group includes contaminants like Pesticides, Pharmaceuticals and EDC’s;
  • Disinfection by-products – Mostly formed with the reaction of a disinfectant with organics present in the water. Trihalomethanes (THMs) are also environmental pollutants, and many are considered carcinogenic.

Differentiating different types of contaminants in municipal waste water and then broadly outlining treatment options.

Each contaminant group is targeted with a different technology or a series of technologies to achieve the removal rating required. In Fig 1 contaminants are linked to the treatment to explain the multi-barrier approach used in Re-use applications. The Log removal examples are used only as explanation and could vary from different studies.

Contaminant Treatment
Suspended solids Sand filters
Microbiological Ultra-filtration
Inorganic salts Reverse Osmosis
Micro-organics GAC & AOP (Advanced Oxidation)
Disinfection by-products GAC

 

What is the core technology for Municipal re-use projects?

Evaluating main contaminants and trying to understand what the most critical component of a Re-use process is form a crucial part of selecting the right technology for a municipal process. For example, ultra-filtration plays an important role in removal of the microbiological contamination. The viruses, bacteria and protozoa provide immediate health risks that need to be addressed as a priority. In any treatment process ultra-filtration will ensure removal of the microbiological contamination as well as finer suspended solids to ensure water quality.

Minimum requirements for an ultra-filtration membrane filtration system:

  1. Filtration membranes must have a nominal pore size not greater than 0.1 µm.
  2. Membrane material must be PVdF and both membrane material and module components must be chlorine resistant.
  3. Membrane material and module components must have NSF certification.
  4. Filtration must be effected from outside to inside of the hollow fibre membranes.
  5. All sequences must be fully automated and suitable for unattended operation, including but not limited to backwash, maintenance wash (MW), membrane integrity monitoring and clean in place (CIP) sequences.
  6. The membrane system must include a membrane integrity test that is capable of demonstrating that the membrane system is achieving a minimum of 4 log removal of particles < 3 µm.