Scottish Water trials earthworms & fleas to treat wastewater

The technology Scottish Water is trialling with earthworms replicates a process which happens naturally within the soil.
The technology Scottish Water is trialling with earthworms replicates a process which happens naturally within the soil.
The objective of the INNOQUA project is to provide a decentralised , ecological wastewater treatment for use in rural communities.
The objective of the INNOQUA project is to provide a decentralised , ecological wastewater treatment for use in rural communities.

The EU-funded INNOQUA project aims to protect and improve the quality of natural water resources by demonstrating that nature based-processes can treat wastewater. One project partner, Scottish Water, is trialling the use of earthworms and water fleas.

Currently, around 2.5 million people globally do not have access to sanitation facilities, so the development of an accessible method to treat wastewater is of vital importance. The objective of the Innoqua project is to provide a decentralised, ecological wastewater treatment for use in rural communities like Littlemill, as well as industries such as agriculture and aquaculture.  

On-site sanitation The Innoqua system is being trialled in 11 countries including Scotland. It is an ecological on-site sanitation system which integrates different technologies and modules, such as the Lumbrifilter which uses earthworms and the Daphnia filter, which uses water fleas. Other technologies include microalgae based biosolar purification and UV disinfection. Due to its modular configuration, the Innoqua system addresses the water treatment needs of decentralised facilities, water stressed communities, rapidly expanding cities and industries both in developed and developing countries. 

Scottish Water pilot A pilot-scale Lumbrifilter and Daphnia filter were installed by Scottish Water at the Littlemill Wastewater Treatment Works as part of the Innoqua project. The system was installed in October 2019 and is being tested until May 2020.

The location of Littlemill in Nairnshire for the 12-month Scottish Water pilot was chosen by its Research and Innovation team based on size, the remote location and exposure to the Scottish climate, to test the reliability and adaptability of the technology. Demonstration facilities have been installed at sites in ten other countries, as part of the Innoqua project, to test their performance under different conditions and in combination with other nature-based solutions. 

The Littlemill pilot will run alongside the existing treatment plant to make sure the local environment continues to be protected, and Scottish Water has been proactively engaging with residents to inform them of the pilot, discuss plans and answer any questions. Earthworms/lumbrifilter

Lumbrifilters, also known as vermifilters or microbial-earthworm ecofilters, are engineered natural systems, based on the interaction between earthworms and microorganisms, in which earthworms degrade and homogenise organic wastes, increasing their surface area and facilitating subsequent bio-chemical degradation of pollutants by the microbial biofilm established on a  filter bed.  

Project manager, Anna Baran from Scottish Water’s Research and Innovation Team, said: “The technology we are trialling basically replicates a process which happens naturally within the soil, but we are using it to clean wastewater. The first stage of treatment involves a tank filled with earthworms – the worms eat the larger particles of organic matter in the wastewater, before it is added to a second tank containing water fleas and microalgae which remove the finer bits of organic matter.”

More effective These vermifilters have been used to treat both domestic wastewater and industrial effluents. The treated effluent is suitable for discharge to the environment, reuse or further treatment. These microbial earthworm ecofilters (MEEs) have been shown to provide more consistent treatment performance than conventional biofilters that do not include earthworms. 

Depending on the type of filter media used, they are also capable of removing or transforming nutrients, and reducing pathogens – they also produce little excess sludge. The Innoqua lumbrifilters are designed to treat primary (settled) domestic sewage, although several project sites have successfully experimented with dosing whole (screened) sewage without settlement.

Design concept  Vermifiltration systems all follow similar design principles:  •     Filter media are built up in a series of layers – normally of increasing particle size with depth. These layers may be mineral (sand, gravel or man-made equivalents) or organic (compost, bark, sawdust);  •     A distinct uppermost ‘bedding’ layer is normally included, comprising an organic substrate or an organic matter rich soil to suit the requirements of epigeic earthworm species such as Eisenia fetida, Eisenia andrei, Perionyx sansibaricus or Lumbricus rubellus. Vegetation is sometimes established in this top layer;  •     Wastewater is introduced to the top of the filter using a distribution system. Wastewater can be introduced as greywater, blackwater or (primary) settled sewage, each of which requires different filter media, depth and operating volume to optimise treatment;  •     Wastewater percolates through the filter bed, where treatment takes place in an established biofilm as it does in aerobic trickling filter systems. Earthworms graze on the microbial biomass and solids introduced in the wastewater which moderates the microbial community and helps to maintain aerobic conditions. The treated effluent collects within the lowest gravel layer or a separate sump, from where it may be collected for discharge, further treatment or re-use;  •     The filter bed requires little maintenance, since a healthy earthworm population will maintain a network of channels throughout the medium. The surface may eventually become clogged with earthworm casts that can be harvested for re-use in agriculture or horticulture;  •     Vermifiltration systems require no external power, although pumps are commonly used to introduce wastewater and/or remove treated effluent in experimental systems. They may be operated as single units, or as multiple units in series – depending on site and wastewater-specific circumstances. 

The bedding material type, filter media type, earthworm species, type of wastewater, temperature, pH, hydraulic loading and other factors can interact to affect the efficiency of the system.

Various media have been trialled, including sawdust, coir, bark, woodchips, gravel, glass and clay balls – with variations in the filter bed type and height influencing the distribution of COD. Aerobic-anoxic microenvironments can then form, which impact on nutrient removal efficiencies

Lumbrifilter’s treatment efficacy Previous studies have examined the treatment efficacy of vermifilters by measuring standard characteristics in wastewater before and after filtration. These characteristics typically include BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), total phosphorus, total suspended solids (TSS), pH, total nitrogen and ammoniacal nitrogen. 

High BOD and COD removal rates have been demonstrated (up to 98% and 80% respectively), but nutrient removal efficiencies depend very much on the design and operation of the system, as well as the required degree of treatment. 

There is a potential for filter bed media to be selected with specific phosphorus-adsorbing characteristics, while long HRTs in well-aerated media will improve nitrification (reducing ammonia concentrations in the final effluent). Influent feeding regime also influences treatment efficiency, with intermittent feeding more likely to allow stable aerobic conditions to develop within the filter bed, reducing potentially harmful ammonia impacts on the earthworms by encouraging nitrification. 

It is thought that the reduction of pathogens (faecal coliform, total coliform, faecal streptococci, salmonellae, E. coli) in vermifilters is mainly due to the action of enzymes secreted within the intestines of earthworms, and within their secreted mucus. Temperature within the vermifilter is extremely influential on pathogen removal, since it can be directly related to earthworm and microbial activity.

Daphnia filter Daphnids or water fleas are small crustaceans, usually found in lakes, rivers and other suitable freshwater habitats around the world. Daphnia magna is a common species that is widely used for ecotoxicological studies. It has a broad natural distribution, having been identified from Russia to India and South Africa.

Other species occupy similar ecological niches in countries as diverse as Australia, Mexico and Sri Lanka. Daphnia magna favours a water temperature of around 20°C, although its distribution means that it can adapt to a broad temperature range. 

Daphnids are filter-feeders that consume small particles suspended in water. This feeding mechanism does not discriminate between organic and inorganic particulate material. Daphnia magna can filter particles between 2 and 30 μm under experimental conditions.

Design concept  The natural filter-feeding characteristics of daphnids mean that they can remove suspended solids that may not normally settle during primary clarification of wastewater. This feeding mechanism also removes some bacteria which suggests that Daphnia could be used both for clarification and disinfection of treated wastewater as a nature-based tertiary treatment. 

Daphnia are sensitive to some wastewater characteristics, including ammonia, nitrite and heavy metals, which means that any system using these organisms must follow initial primary and/or secondary wastewater treatment. Within the Innoqua system, the Daphnia filter is always located downstream of a Lumbrifilter and the final effluent is either discharged to environment, or passed through a UV disinfection unit, where water re-use is planned. 

The Innoqua Daphnia filter aims to couple the filtering capabilities of daphnids with the nutrient transformation and removal capabilities of microalgal/bacterial biofilms. The Daphnia filter is not expected to provide the level of disinfection necessary for direct water reuse, however, reductions in turbidity resulting from removal of suspended solids will improve efficiency of the downstream UV disinfection stage. 

The Daphnia filter is configured as a tank inoculated with local species of Cladocera (usually Daphnia magna) which are free to move up and down the water column. Internal weir and baffle mechanisms ensure that the daphnids remain within the reactor, whilst additional surface area is provided for the establishment of the microalgal/bacterial biofilm.

How efficient is the Daphnia filter? Various previous experiments have investigated the clarification potential of daphnids.  One established a series of 1 m3 sedimentation tanks, with each followed by a Daphnia filter of the same size, hosting populations of Daphnia magna. Each tank operated under a hydraulic retention time (HRT) of one day. 

The study found that the whole sedimentation / filtration system delivered suspended solids removal of up to 99%, when Daphnia magna populations ranged from 10–50 organisms per litre. A laboratory-scale study with a similar arrangement removed >90% of the suspended solids, although ~60% of this removal was attributed to sedimentation, with the remainder due to Daphnia filtration.  In the Innoqua system, the initial sedimentation or solids-removal phase would take place in a Lumbrifilter. 

Looking to the future With all demonstration sites now in operation and the Littlemill trial expected to finish in May 2020, Anna Baran is looking to its potential for impact on a global scale. “This is a really exciting project for us to be part of and has the potential to have a real impact on the way wastewater is dealt with around the world, particularly in developing countries.”