New approaches to suspended solids removal
Peter Minting looks at two new filtration methods introduced at the World Filtration Congress by scientists from Germany and Hong Kong.
Zebra mussels (Dreissena polymorpha) have often been described as a pest of potable water treatment plants as they can congregate around intake pipes and reduce flow. But researchers Ralph Kusserow and Dietrich Uhlmann from Dresden University of Technology have found a way in which zebra mussels could be used help treat sewage effluent.
Though sophisticated sewage treatment works (STWs) are able to remove a high amount of dissolved organic substances and nutrients, their effluents may still have a high concentration of suspended solids (SS).
Many large STWs have final polishing ponds with a low flow rate and a typical residence time of around two days. Dense growths of algae can add to the SS burden and contribute to consent standard failures.
A search was launched for a biological solution to this problem as there are many organisms which can thrive on filtering out small organic particles. Mussels are known to be able to filter large volumes of water very rapidly, so in this case a system was set up to test the filtration rates achieved by the common zebra mussel. As an experiment two reactors with three layers of polypropylene matting were set up, one with empty mussel shells as a control and the other containing 5000 live mussels collected from their natural habitat. Empty shells were attached to the mats one reactor using silicone rubber and live mussels allowed to attach naturally in the other.
A maximum flow of 80l/hr was passed through each tank, with a residence time of around 40min. Homogenised activated sludge was added to the flow to test the ability of the mussels to filter sewage effluent.
The results showed live zebra mussels were capable of reducing turbidity by around 40% when the SS content was 40mg/l. The mussels were seen to filter effluent with a particle retention rate of around 85% at rates of 2-70ml per mussel per hr (around 20ml/hr with an SS content of 40mg/l).
Within the mussel's preferred temperature range of 10-200ºC the filtration rate was not temperature dependent but below 50ºC and above 200ºC filtration rate declined significantly.
Experiments with the polypropylene mats showed them to be an excellent medium for the attachment of the mussel's free swimming larvae if suspended in a water body colonised by the animal. After suspension for a period of three to five months, juvenile mussels had grown on the mats at densities of up to 70,000/m2.
Zebra mussels could therefore be used to help polish final effluent in settling ponds, as long as extremes of temperature can be avoided and there is a degree of control over the influent SS levels.
Dr W.O Leung of Waste & Environmental Technologies in Hong Kong introduced a technique for the treatment of muddy waters produced by construction activities. Next to oil-related waste and sewage, construction run off is the biggest cause of urban river pollution in the UK, according to the Environment Agency (EA). The Agency's pollution prevention officer, Teresa Brown: "The construction and demolition industry is responsible for causing over 600 pollution incidents each year, many of which could be avoided.
The conventional method of handling muddy water is to pass it through rectangular tanks for sedimentation. Occasionally alum is added to the tanks by hand to boost the sedimentation rate. However, this is unlikely to result in a waste which meets effluent standards. The suspended solids (SS) content of wastewater from foundation drilling is often around 20,000mg/l but a common discharge standard in Hong Kong or the UK for construction site effluent is around 30mg/l. Clearly there is a need for an effective treatment system. Drilling and tunnelling activities generate a great deal of muddy water as water is used as a lubricant and coolant.
Dr Leung's process, the WetSep wastewater filtration system, has been developed in Hong Kong as a low-cost, low-maintenance method of dealing with this kind of pollution.
The WetSep is designed to remove SS using an 'impinging stream' reaction and corrugated plate interception. The system takes advantage of centrifugal forces and gravity. At the inlet of the system a flocculant (anionic polymer) is added followed by a coagulant, in this system polyaluminium chloride (PAC). The impinging stream reaction works by the creation of a spiral flow, resulting in three separate streams according to density. Oil rises to the top of the chamber, SS to the bottom and water leaves via a central path.
After the impinging stream reaction the water enters a second chamber under gravity flow, and passes through a stack of closely separated corrugated plates. This removes any remaining flocs or oil residues. The settled flocs and oil from both stages of the treatment process are discharged through hoppers at the base of the unit.
Three types of construction effluent were treated with the WetSep system, run off from house building, from tunnel grouting and bore piling. In all three cases WetSep removed 99.8% of SS. In the case of the bore piling effluent SS was reduced from 25,000mg/l to 30mg/l when treating a capacity of 100m3/hr. Run off from house building was reduced from 10,400mg/l to 20mg/l.
According to Dr Leung the system is therefore suitable for this kind of application. The key is simplicity as the system depends on gravity and has no powered moving parts apart from the coagulant injection system's metering pumps. An additional delivery system will of course be necessary to deliver wastewater from around the site in question.