Dr Evangelos Petropoulos
Premier Tech Water & Environment
There are approximately 5,000 off-mains small sewage discharges (SSD) in England annually. These predominantly involve small treatment plants (< 50PE (people equivalent)), often subjected to 12566-3 or 12566-7 for secondary and tertiary treatment respectively (or 12566-1 in the case of septic tanks). Set out as requirement for discharge permits by the Environment Agency (EA), small sewage treatment plants focus on the removal of BOD, TSS and NH3/4-N (organics, suspended solids and ammonium nitrogen respectively); total Nitrogen and Phosphorus also require treatment at sensitive watercourses.
Treatment-wise, engineered treatment plants are unable to completely purify the wastewater, hence, residual pollutants/contaminants in the effluent are expected. Although this may not be a major issue if discharge is to the ground (infiltration bed), for surface water discharge, partially treated wastewater may come in direct contact with communities, as well as aquacultures (i.e shellfish) raising public health concerns.
Most of these plants have limited disinfection capacity, hence, pathogenic microorganisms may accumulate in waters over time, especially if replenishment is slow. Water quality in such environments involves microbiological standards on the basis of indicator organisms that include enteric bacteria (i.e. E. Coli, Enterococci), Helminth cysts and/or other.
Raw wastewater contains a pathogenic population ≥106 CFU/100ml (as E.Coli), with septic tanks removing ≥50%.1 Secondary treatment is expected to reduce that considerably, reaching levels between 3×103-105 CFU/100ml (as E.Coli).2,3
To further reduce the pathogenic population, options with incorporated disinfection or tertiary treatment to polish the effluent is required. Such mechanisms are based on chlorination (or similar), ozonation, UV irradiation and/or filtration procedures commonly seen in large treatment assets catering for a large population. However, in the case of SSD, such approaches make CAPEX/OPEX extremely high.
For smaller installs, (i.e.<100 PE), simple, cost-effective and hazard-free options are essential. To achieve this, filtration remains a key strategy, involving passive (zero-electricity/minimal CO2e) filter beds, that work without chemicals and without the need for intensive maintenance.
Filter beds, often rich with coir material, peat, sand or similar, have been previously proved effective against the removal of pathogens. Current market trends, focusing on carbon footprint and sustainability, render peat an environmentally concerning approach; with the low resiliency/reduced stability, and limited hydrophilicity being the main contributing factors. Sand is a traditional disinfection method that can be used as polishing (tertiary); however, limited nitrification, variations in the mineral composition and intrinsic porosity render this method only suitable for tertiary, while increased maintenance, often involving backwash increases operational complexity.
Coconut by-products are widely used in the water industry (i.e. activated carbon). Filter beds of coir are also common and are used for both secondary (after septic tanks) or tertiary treatment (after sewage treatment plants). Such options, i.e. the Ecoflo system from Premier Tech Water and Environment, bear a combined ability to treat wastewater in terms of BOD, TSS and NH3/4-N (≈5.0mg/L for all), as well as disinfect, delivering an effluent ideal for discharge to bathing zones (inner and outer) and/or in proximity with aquacultures.
The removal efficiency is dictated by filter surface loading, porosity, and filtration height. Independent trials from Veolia in France (CSTB, Nantes FR) showed that the Ecoflo system can deliver a 99.2% pathogens’ inactivation (2.2log) in a secondary format, while in-house testing (more than 200 samples) showed that as tertiary (after a sewage treatment plant such as a Premier Tech Rewatec Solido Smart), the Ecoflo system can ensure E. Coli ≈500 CFU/100 ml (or up to 4.0log inactivation (99.99%)).
The above highlights the need for clarity in the bathing/coastal water quality which is often at risk due to partially treated discharges. Although SEPA have recently addressed this with the recent guidelines (Microbial Discharges Regulatory Method WAT-RM-13 v5)4, Natural Resources Wales and the Environment Agency have not yet clarified what the SSD effluent should be in terms of pathogens. This can become a major concern due to the anti-microbial resistant organisms that lately prevail.
Overall, pollution dynamics show that products like the Ecoflo system can sustainably ensure the quality of water for future generations.
1. DOI: https://doi.org/10.3390/w13091190
2. DOI: 10.1080/15287394.2014.989626
3. DOI: 10.1007/s11270-016-3157-8
4. SEPA 2022, Regulatoty methods: Microbial Discharges Regulatory Method WAT-RM-13 v5