Algae ponds can clean our dirty water
A beautiful glistening pond catches the warm Eastern Cape sun, surrounded by green hills and a dairy farm in the distance. Water runs from this pond into smaller ponds.
You would be forgiven for thinking you can swim in the water in this pond.
This is biology at work in the form of the integrated algae pond system – turning sewage into clean water.
The prospect of deploying this technology around South Africa has been met with hesitation, because the water quality is inconsistent. Tests carried out over two years from 2012-2014 showed that the pilot had not only been incorrectly configured but also lacked components that would improve the overall performance of the plant.
These problems with the pilot plant should not impact the reality that this technology has far-reaching benefits for South Africa.
The reality is that one in nine people globally do not have enough access to clean water and sanitation, while 3.4-million, mainly in the developing world, die from water, sanitation and hygiene-related illnesses every year, according to the World Health Organization. As a water scarce nation, South Africa is reaching the stage where demand for water will outstrip supply.
In some instances, this is already the case and the situation will continue to deteriorate if nothing is done. Part of the problem is that the South African population continues to grow, while new infrastructure struggles to keep up.
A possible environmentally friendly solution to waste water treatment lies in Grahamstown in the Eastern Cape. This system – developed by University of California, Berkeley, researchers, – has been successfully implemented as a pilot plant in Grahamstown, through the Institute for Environmental Biotechnology at Rhodes University.
At the time of commissioning in 1996 it was touted as an eco-friendly technology capable of the highest degree of waste water purification at the lowest cost with minimum maintenance. This is still the case today.
The process begins with sewage flowing from municipal sewerage pipes into an underground chamber, where anaerobic digestion begins. Anaerobic means “in the absence of oxygen”. In the oxygen-free chamber, bacteria break down organic particles like complex carbohydrates and proteins found in sewage to produce simple sugars, nitrates, phosphates and various other compounds that easily dissolve in the water.
Biogas, a gaseous product of this process which includes methane, rises from the bottom of the digester, passing through the water column to form mostly methane at the top of the underground chamber. Depending on the quantities generated by and captured from the digester, methane, which is continuously produced, can be used for heating, cooking and electricity generation or directly used to run the waste water treatment plant, reducing electricity costs.
The contaminated and nutrient-rich water, which still requires treatment, is then channeled into the primary facultative pond, which contains algae.
This type of pond is special as it contains an oxygen-rich and an oxygen-starved layer, which means microorganisms simply move to the layer where they are most comfortable and able to survive. This is the first and most prominent pond to greet you at the institute. Not only is it aesthetically pleasing, it appears to sit on a pedestal overlooking a series of smaller ponds. In accordance with the design, the primary facultative pond rests directly on top of the anaerobic digester, essentially working as a single unit, but each carrying out two distinct activities.
The facultative pond acts as a buffer system where algae begin to take up the excess nitrates and phosphates, initiating nutrient removal from the water emerging from the oxygen-free digester. Any organic particles that escape the digester and debris that settles from the surface of the primary facultative pond, are anaerobically digested at the base of this pond.
The water leaving this section of the waste water treatment plant then moves into the first of two smaller ponds, called high-rate ponds. These are two oblong shaped ponds with a wall along the length of the pond dividing it in two equal halves. Each rounded end of the pond contains baffles to direct the water and maintain an even flow in the pond.
Flow of water is generated by a paddlewheel on one half that circulates the water in the entire pond. These ponds – working in series – take advantage of algae to treat the water further. The algae take up the nutrients, such as nitrates, phosphates and potassium, and through photosynthesis produce oxygen. Photosynthesis makes the water more alkaline.
This alkaline, oxygen-rich environment is toxic to anaerobic bacteria which die, and exposure to the ultraviolet rays in sunlight disrupts the DNA in other undesirable microorganisms, essentially resulting in a naturally disinfected water containing algae.
This algae biomass is then separated from the water in stationary settling ponds, which look like large boxes filled with water. Here, the water is captured and sent back to the municipal waste water treatment works, while the algae biomass is left to dry on drying beds.
To complete the waste water treatment, a maturation pond is required. This is a pond where the water is allowed to “mature” prior to discharge, where components such as pH settle to neutral, oxygen content declines to the levels found in rivers and the nutrient content settles below a specific threshold, regulated by the department of water affairs, to ensure no detrimental impact to the environment. Unfortunately, the Institute for Environmental Biotechnology does not have this maturation pond, which is why the water has to be sent back to the municipal waste works where it treated for discharge into the environment.
However, the integrated algae pond pilot system in Grahamstown still serves as an example of a sustainable, “green” waste water treatment process. It is “green” because the carbon dioxide released by the system is then taken up through photosynthesis in the numerous ponds, reducing carbon dioxide production, which is implicated in climate change. The algae biomass produced by the process can also be fed into the anaerobic digester with the waste water to boost methane generation. Unlike other waste water treatment processes, the integrated algae pond system does not require chemicals to disinfect the water. The pond system also produces potentially profitable by-products such as algae for bio-fertilisers and methane gas for heating and cooking.
These ponds work together, ingeniously exploiting normal algae and bacterial activities, to treat waste water. With more of these integrated algae pond systems, we may be able to treat the dirty water and sewage that comes with a growing population.
Prudence Mambo attends Nelson Mandela Metropolitan University and was formerly at Rhodes University.