David Helicon, Principal Waste Water Scientist at South West Water, has developed an industry leading solution to de-watering activated sludge. The result is a more desirable agricultural fertiliser and improved efficiency at the treatment works.
Activated sludge is a real problem for the wastewater industry. Unlike primary sludge, activated sludge is difficult to de-water. This makes activated sludge less stable than primary sludge for use as an agricultural fertiliser, meaning it doesn’t store well before spreading.
What this means is that activated sludge cake, while waiting to be spread, can run down the farmer’s field towards a watercourse. This can potentially have a disastrous impact on the wildlife in and around the watercourse.
This presents the industry with a problem. Normally water companies mix activated and primary sludge together, using specific ratios, so that stable sludge cake can be produced.
Activated sludge cake is an ideal soil conditioner containing essential nutrients for crop growth, especially phosphorous. Phosphorous can also be derived from non-renewable sources, so recycling sludge cake in agriculture is highly sustainable.
To summarise, producing stable sludge cake for use as a fertiliser is the most sustainable method of disposal.
At Cornborough Waste Water Treatment Works (WWTW) in North Devon, a site which serves 41,000 people, the importing of primary sludge is prohibited by planning restrictions. This means that the normal industry practice of mixing primary and activated sludge isn’t possible at Cornborough WWTW, presenting the site with a problem for how to make use of the activated sludge.
In 2013, a centrifuge was installed to reduce sludge cake loads from the site, but because of the inconsistent quality of cake, this was never achieved. To help form the sludge cake, a very high concentration of de-watering polymer is dosed into the centrifuge.
The new industry leading idea, to increase dry solids achievable for activated sludge, was developed in 2017. The idea is simple: activated sludge is a soup of micro-organisms, which makes it difficult to de-water. Micro-organisms contain a high concentration of water and by disrupting the cell structure, the water can be removed from the cell; meaning a higher rate of de-watering can be achieved.
Based on this idea, a crown destructor unit, which disrupts the cells through rapid pressure changes, was installed at Cornborough WWTW. This unit was plumbed into the centrifuge feed tank and sludge pumped from the base of the tank through the destructor and returned to the top of the centrifuge feed tank. This configuration means that the centrifuge operation does not rely on the destructor being on-line.
The destructor is situated in a shipping container and sits next to the sludge tank, feeding the centrifuge. In the trial, the destructor ran continuously from April 2018 to July 2018.
The results speak for themselves and exceeded expectations. Before and after microscopic imagery shows how the sludge structure is fundamentally changed, transforming the random bacterial floc particles into an even liquid that can easily be pumped into the centrifuge. The changes are also visible to the naked eye, with a strong change in viscosity and smoothness.
According to literature, the dry solids achievable for activated sludge is in the region of 18-20%. However, with the use of the destructor unit, the dry solids in the activated sludge improved to 24%, which is on par with primary sludge cake. This means the end product is much more stable and so the sludge cake is more readily accepted for agricultural use.
This idea has had major operational benefits too. The sludge cake volumes reduced from 7 bins to 5 bins a week due to the reduction in water, reducing the need for operator involvement over the weekend. This reduced volume also makes transportation easier. The polymer dose was also reduced by over 20% – this reduction alone means the project cost will repay itself within just one year.
A surprising benefit has also been that prior to installation of the destructor, the centrifuge was affected by varying saline infiltration which required a varying polymer dose throughout the day. By bursting the cellular structure, this has limited the ability of sulphate-reducing bacteria to form hydrogen sulphide. This is a significant benefit, as this gas can adversely affect de-watering and reduces the health and safety risk associated with the accumulation of the gas.
The process also resulted in a net energy decrease. Although the destructor unit consumes electricity, this energy use is offset by running the activated sludge process at the correct control set-points.
The use of the destructor to break cell walls has produced an industry leading de-watering process, so activated sludge cake can be just as desirable as an agricultural fertiliser as primary sludge cake. This means there is now a way for the nutrients in the cake to be recycled sustainably and ensures the poor sludge cake doesn’t end up being reprocessed. More than this, it has reduced site costs in the long term, while decreasing the need for operator intervention.
The destructor unit is now a permanent part of site operations and shows how important it is for the water industry to de-water sludge effectively.