Alternative coagulants: swapping metals for plant-based alternatives

Alternative coagulants: swapping metals for plant-based alternatives

Becci Bragg, from the Science and Optimisation team at South West Water, demonstrates how using more environmentally friendly plant-based alternatives to metal ion coagulants are being introduced and tested at wastewater sites.

Over the last 10 years, the use of metal ion coagulants has been increasing within wastewater treatment. This has partly been driven by tightening regulations surrounding phosphorus, which if present in rivers and streams, can lead to serious ecological problems through the excessive growth of algae.

Within the South West Water catchment area, the use of metal based coagulants also help with extreme variations in loading associated with the local tourism industry, where the population can nearly double in certain locations during the summer months.

While the use of metals in wastewater is strictly regulated and monitored through operator self-monitoring (OSM) methods, there are several environmental and local concerns associated with its use.

The use of metal is non-renewable, often travelling long distances. Aluminium is also toxic to fish when present in certain quantities and pH conditions.

Drinking water within the South West Water catchment is largely sourced from moorland rivers and reservoirs and is therefore typically soft, with low pHs and very little buffering. As a result, the use of aluminium based coagulants typically depresses the pH outside of the optimal range for sewage treatment and additional buffering (carbonate or bicarbonate ions) is therefore required.

Added to that, wastewater treatment sites are often widely dispersed and discharge into small inland watercourses, meaning discharge permits can have tight limits.

Organic materials have long been used to clarify drinking water for centuries, dating back as early as 2000BC. One of the most effective and well documented organic coagulants is the seeds from the Moringa oleifera tree, a fast growing, drought resistant tree native to tropical regions like India.

Studies dating back from the early 70s demonstrate that the seed kernels are highly effective in reducing suspended solids in turbid waters. The plant is widely cultivated for its seeds, which are pressed to extract a type of oil known as Ben oil.

These seeds from the Moringa oleifera tree work well as a coagulant due to the positively charged, water soluble proteins, which bind to negatively charged particles, in the same way that the metal coagulants work.

However, these Moringa seeds are not readily available here in the UK, and the long transport route defeated the teams aim for a truly sustainable alterative.

The science and optimisation team set out on a mission to find seeds readily available here in the UK that might work similarly to Moringa.

Seeds used in commercial oil production, were selected for initial laboratory tests, including hemp, pumpkin, linseed and sunflower seeds. Using these seeds against a commonly used control metal coagulant, polyaluminium chloride (PACL), a lab trial began in the search for a viable plant based alterative coagulant.

Initial trials quickly ruled out the dried ground hemp, linseed and sunflower seeds, as they proved too oily to produce a useful coagulant and removal of these oils would prove costly.

Although there was little evidence pointing towards their effectiveness in wastewater, soybeans were introduced to the trial as they too presented some similarities to Moringa.

Soy and pumpkin seeds were compared to our most frequently used PACL coagulant to treat effluent from a plastic media biological percolating filter.

Trials indicated that soybeans were the most effective coagulant at removing solids from the turbid waters and surprisingly, they were found to be even more effective than the control metal coagulant, PACL. Pumpkin seeds were also effective, and in some doses, found to be more effective than the control PACL.

This development was highly encouraging, given that soy growth in the UK has been on the increase, making it a highly viable and sustainable source.

Even more encouragingly, the trial suggests that the sludge formed using the seeds (particularly the soy) was far denser and has the potential to form a much more stable sludge blanket than that created with PACL.

This could provide far greater process stability when the wastewater treatment works experiences rapid changes in flow. In turn, this means less metals entering the environment and a more digester friendly sludge, which could potentially increase gas yields.

South West Water spends approximately £1.5million annually on metal coagulants and this study provided an influential starting point for further investigation. While these early results are highly promising, converting what has been found in the lab to a viable large-scale operation is a challenging one.

In collaboration with coagulant provider Taytech Environmental Limited, a different organic product (tannin based) sustainably sourced from the tree known as Acacia mearnsii, was found commercially available for use as a coagulant, and named QP33.

Acacia mearnsii is a fast growing but short-lived tree, found in many warm countries across the globe. While the tannin is not native to the UK, the ability to source this product on a large scale readily from Taytech Environmental Limited presented an ideal opportunity to test the product outside of the laboratory and at wastewater treatment site.

The jar tests for this product gave impressive results, performing at least as well as the aluminium coagulants they were tested against.

The QP33 was put through a full-scale trial at South West Water’s Callington Wastewater Treatment Works (WWTW), located in East Cornwall, an area with an extensive mining history, with both banks of the river Tamar having supplied much of the worlds copper and tin, at times, over the last 4,000 years.

The site has periodically suffered with denitrification in the single final settlement tank (FST), resulting in marginal suspended solid failures. A trial of dosing Poly Aluminium Chloride into the FST feed sump showed that this could improve performance, however the pH of the effluent was depressed and the Environment Agency require a tight aluminium permit to be applied. Metals in the receiving watercourse have the potential to be elevated, due to the previous mining activity in the area.

Performance between the alterative (QP33) and metal based coagulants when used on the site show comparable results, but with some added advantages for the organically derived QP33. For instance, when using the organic coagulant, the use of sodium carbonate reduced, as QP33 does not depress the pH.

South West Water is now in the process of trialling QP33 on a larger scale at Hayle WWTW. The site has an anaerobic sludge digester and the plan is to see if gas yields can be increased from using this organic coagulant.

The success of the trials has been hugely encouraging. There is potential that as the trial progresses, that metal coagulant use can be greatly reduced across the South West Water wastewater network, improving sustainability, safeguarding ecosystems, reducing chemical additions and potentially increasing gas yields.

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