Sewage worms become fish feed

The Netherlands produces a great deal of wastewater, which is largely processed in wastewater treatment facilities. ‘Wastewater treatment in the Netherlands is relatively efficient, but has the disadvantage of generating a large amounts of sludge’, says Tim Hendrickx. ‘Because the sludge contains many contaminants, it must first be processed before it can be used for other purposes, such as fertiliser.’ Because this purification is costly, in the Netherlands the sludge is often settled, dried and incinerated. However, this process is also relatively expensive.

But now a worm species, Lumbriculus variegatus, which lives everywhere in the Netherlands in ditches and canals, could reduce the costs of sludge processing. The animal places few demands in its environment: it needs only a plastic screen on which to attach itself, some sewage sludge to eat and oxygen-rich water at about 15 °C. This enables the worm to digest large quantities of the brown substance. ‘The worm is attached to a screen that separates clean water from sludge. The worm stands with its head in the sludge and its tail, with which it breathes, in the oxygen-rich water’, explains Hendrickx. ‘In this way, you can also easily separate the worm castings from unprocessed sludge.’ Ultimately, the worms break down 15 to 75% of the dry matter in the sludge and convert the remainder into compact worm castings. As a result, the volume decreases significantly. Because the animals metabolise some of the sludge and use part of it for their own growth, the sludge volume decreases even more. As Hendrickx calculates, ‘If you assume 20% decomposition, you can ultimately reduce the sludge volume by 70%. In this way you can save a great deal on both the incineration and transport costs for the sludge.’

A beneficial side effect of this purification method is that you create a significant amount of worm biomass, which has many applications, such as fish feed. ‘The worms have a high nutritional value and they are very suitable as live feed for fish farms’, Hendrickx believes. ‘Unfortunately, some people find this objectionable, because including waste in the human food chain is still a taboo topic.’ In cooperation with the Aquaculture and Fisheries group, research is being conducted into possible uses for worms that have been grown in both sewage sludge and in cleaner sludge that is produced by the food industry. ‘If they succeed in finding a good use for the worms, the investment in a worm purification system, approximately €400,000, would become much more cost effective’, says the doctoral candidate in closing. / Hans Wolkers

Tim Hendrickx defended his doctoral thesis on 16 June; his thesis supervisor was Prof. Cees Buisman, AgroTechnology and Food Sciences, Sub-department of Environmental Technology.