In good health part 2: wastewater treatment options

What are your site specific options if you plan to install a waste water treatment system in NI or ROI?

The main points to remember from my previous article are that your tank needs to be checked at least annually and de-sludged when necessary depending on its size and the number of people in the house. You will also need to carry out upgrades if your tank is inspected and deemed to require these (grants are available but in ROI only if your tank fails an inspection and you cannot request one at present). Upgrades and repairs are usually more cost effective and have a lower carbon footprint than replacing the unit. Before putting in a new system a fresh site assessment must be carried out and planning permission obtained. This is also required for green field sites where the sewage treatment system will be an integral part of your overall planning application.

Site assessment

The site assessment is the first stage in the decision making process, determining size, shape, topography, soil and subsoil permeability and depth to bedrock or groundwater. These factors will determine whether or not you will be able to get planning permission for a standard system. The site assessor in ROI may be from the council, on a list of pre-approved assessors or simply a qualified engineer – accreditation is recommended but not strictly necessary. In NI the householder can carry out the BS6297:2007 percolation test and present the results to the authorities (NIEA) but there is a warning in the application form that if any results are presented in an untruthful manner the applicant can face a £20,000 fine. Said assessor will carry out percolation tests and dig a trial hole to investigate the site characteristics. If the site assessor also sells a particular product, be aware of a possible bias in the recommendations proposed and always explore alternatives before purchasing. If the soil is heavy and full of clay, a standard approach may not be suitable. In NI your options include waterless toilets or cesspools.

Dry toilet positioned over collection chamber

Waterless toilets are either self-contained, with an inbuilt composting chamber, or are emptied to an outside compost area or chamber when full. These are dry units that greatly reduce the overall environmental impact insofar as they eliminate toilet water use, recycle humanure biomass to the soil, recycle nutrients for plant growth and reduce sewage contamination of groundwater or surface waters. Dry toilets generally require more direct maintenance as you must compost the humanure on an ongoing basis and this must be done correctly to ensure the pathogens have been dealt with, but on the plus side there is no septic tank to empty every year.

Note that a cesspool, (an underground storage tank that is periodically emptied and brought in for municipal treatment), is not allowed in ROI (or Scotland); in fact the Environmental Protection Agency Code of Practice (ROI) deals only with discharges to ground which means other systems are more difficult to get planning permission for.

However, local authorities in ROI may allow for a zero discharge system on a greenfield site but it depends how open that particular local authority is to this option. Generally if the site is unsuitable for infiltration to ground, then it is deemed unsuitable for development. A pragmatic approach may be adopted for an existing dwelling. There are a number of distinct stages in sewage treatment, namely preliminary settlement or primary treatment; secondary treatment; tertiary treatment and finally, disposal. 

Primary treatment

Primary treatment is the settlement stage whereby the kitchen and toilet solids are separated from the liquid fraction. This is usually in a septic tank, or the initial stage of a mechanical treatment system.

This level of treatment is only ever a preliminary process, so a standard septic tank and percolation arrangement relies upon the percolation area for the actual treatment to occur. Microbes in the soil do the work of treating the effluent to a certain standard before it reaches the groundwater.

Septic tank schematic

After sewer connections, this is the next most common disposal method for domestic sewage, and for good reason: if the soil conditions are good and the system designed correctly it’s a very effective and economical treatment method.

Disposal to water courses is generally not permitted for primary treated effluents. Neither may septic tank effluent be discharged in soils that are too shallow (the groundwater being too close to the surface or the bedrock is high), or where the permeability is either too rapid or too slow because the treatment afforded by percolation will be compromised in such situations.

Site improvements may be carried out to provide additional depth to groundwater or bedrock if percolation conditions are otherwise suitable. In this case, a raised mound percolation area may be built, and is typically pump-fed unless a gravity fall to the raised area is still possible. Where gravels make infiltration through the soil too rapid, then secondary or tertiary treatment is needed.

Secondary and tertiary treatment

Secondary treatment involves the aeration of the effluent so that the overall biochemical oxygen demand (BOD – the food value to microorganisms), and suspended solids concentrations are reduced by about 90%. This lowers the pollution potential of the effluent. If extra treatment of this type is required, the common routes are pump or gravity fed filter media units (e.g. peat), mechanical aeration (usually in the form of packaged systems) or treatment wetlands.

Whilst good at achieving BOD and suspended solids reductions within a small footprint, there are ongoing electricity costs to consider and carbon footprint implications.

Tertiary treatment is an extra treatment stage. Typically nitrate and/or phosphate concentrations are lowered, in addition to further reductions of BOD and suspended solids. Where a gravel soil makes infiltration too rapid, tertiary treatment may be used to make disposal to ground legally acceptable.

In ROI, tertiary treatment may in principle be used prior to discharge to a watercourse, however in practice, although this is permitted in the EPA Code of Practice, the local authorities have a policy of not granting discharge licences to watercourses for domestic dwellings. In NI direct discharge to a watercourse may be permitted under licence after secondary or tertiary treatment.

Tertiary treatment system types include treatment wetlands, willow filters, soil or sand polishing filters or packaged tertiary treatment units

Treatment wetlands is a general category that includes a number of different planted system types, generally divided into gravel and soil based systems. These can be used for either secondary or tertiary treatment depending on the design and application.

Gravel reed beds are further subdivided into horizontal (sub surface) flow systems where effluent enters at one end, flows through the planted gravel bed and discharges at the far end; and vertical flow systems which are pump-fed at top surface level and effluent is allowed to trickle down through the planted gravel medium before gathering at the base for collection and discharge.

Soil based constructed wetlands are shallow planted marsh-type systems. Integrated Constructed Wetlands are essentially a type of soil based constructed wetland, described in separate government guidance in ROI.

With careful design and construction, treatment wetlands provide the right physical, chemical and biological conditions for secondary or tertiary treatment with no electricity consumption.

Willows are listed in the EPA Code of Practice as an add-on to treatment wetlands. Purpose-built willow percolation areas provide enhanced uptake of nitrates and phosphates. Proper design is needed to prevent willow roots filling the distribution pipes and blocking them.

Soil and sand polishing filters are typically pump-fed to provide an even distribution of secondary treated effluent across the filter surface. These function in the same way as a standard percolation area, where aerobic microbial activity within the soil or sand provides treatment of the effluent before it reaches the groundwater. Care is needed to ensure that the correct grades of soil or sand are used. These systems can be used for effluent disposal and as tertiary treatment.

Packaged tertiary treatment systems include packaged media filter units of sand, peat, textile or other media; packaged reed beds (typically considerably smaller than tailored systems); ozone or UV sterilisation systems for killing pathogenic microorganisms in the effluent; membrane filtration units for pathogen and suspended solids removal; and nutrient removal systems that typically target phosphates.

Following contained tertiary treatment such as a treatment wetland or packaged system, effluent disposal by infiltration can be carried out in a smaller area than would be required for a percolation area or soil/sand polishing filter. Zero discharge options.

Zero discharge options

As seen above, in ROI if your site fails the percolation test it is deemed unsuitable for an onsite wastewater treatment system. It’s only with non-Code of Practice systems like integrated constructed wetlands, which have their own guidance document issued by the ROI Department of Environment, and zero discharge willow facilities, which rely upon Danish EPA guidance, where you can begin to find legal options for failed sites. That is, some councils may accept these as viable alternatives; there also has been a precedent for An Bord Pleanála to overturn a refusal.

Integrated constructed wetlands provide zero surface discharge, and due to the very clean nature of the effluent in the final stages of the system, infiltration into even poor soils can be a significant disposal pathway in combination with evapotranspiration through the wetland plants. Zero discharge willow facilities are for soils which have no percolation to speak of or where zero pollution impact is required. When carefully designed and built, the very high growth rate of biomass willow cultivars provides sufficient liquid uptake to dispose of 100% of the effluent by evapotranspiration.

Both options require primary treatment (septic tank). Bear in mind that as with any sewage treatment, all ‘natural’ systems, e.g. reed beds, willow systems, etc. require site specific calculations and design in order to work effectively.

Source separation

An alternative to treatment in the conventional sense is to separate the constituent parts of the sewage for recycling back to the soil.

Urine diversion toilets remove urine within the toilet bowl and pipe it separately for storage and removal for use as an agricultural fertiliser. Waterless urinals may be used by pubs and restaurants as a source separation method, and in sensitive environments this reduces nitrate and phosphate loadings.

Scandinavian faecal separator units, meanwhile, can be fitted within the sewer pipe network to separate solids from liquids. This reduces the overall pollution load to the percolation area or treatment system and allows biosolids to be recycled to agriculture, offering the benefits of a compost toilet with the ease of use of a conventional wc.

Think about it for a moment, if we continue to strip biomass from our soil without ever replenishing it does not make for a sustainable society. Separation technologies have the additional advantage of removing pharmaceuticals (medicine) which are excreted from the body.

Typically these are flushed to sewage treatment where only about half are removed, and the rest end up in our groundwater and rivers and ultimately in many of our drinking water supplies. Phosphorus is another issue. In sewage effluent it contaminates our waterways causing eutrophication of rivers, lakes and streams. In agriculture it is a valuable nutrient which is sourced chiefly from imported mined rock phosphate.

Current projections suggest that phosphate production may peak in coming decades, leaving us with significantly reduced growing power. Unless of course we join these dots and separate out phosphate-rich urine and humanure for returning to agriculture. By harvesting phosphate before it gets into our waterways we cut down on water pollution and build up our soils and our economies.

The infrastructure is relatively commonplace in some Scandinavian countries already and has been used in municipal and one-off house projects with no significant change to the appearance of the wc or bathroom.

The whole point of installing a wastewater system is to treat your sewage to an acceptable standard before reintroducing the effluent to the groundwater. Not returning any effluent and recouping nutrients and biomass for soil building is the ideal solution and therefore the most effective system is the dry compost or source separation system.

Thermophilic composting kills all common pathogens, which is something that none of the other standard sewage treatment systems can boast. Given that our groundwater ultimately ends up back in our taps again (or the taps downstream of us), the cleaner our sewage the safer our drinking water.

Certainly we can use chlorine to sterilise our water, but trihalomethane concentrations (a toxic breakdown product of chlorine dosing in drinking water) already exceed the recommended limits in many ROI drinking water supplies, so it’s best to keep our surroundings as clean as possible in the first instance.

In Denmark, the utility companies are not permitted to sterilise the water supply with chlorine, so they need to ensure that the reservoirs are kept well protected in the first instance. This pride in water quality has led to an annual competition where different parts of Denmark vie with one another for the best tasting water in the country.

Therefore for homeowners who want the best performing system, discharging the cleanest and least polluting water, then a zero discharge willow facility (for nutrient recycling to biomass) or a source separation or dry system (for nutrient recycling to agriculture or the garden) is certainly the way to go.

Carbon footprint

Another consideration is the carbon footprint of your system. In terms of embodied energy (cumulative energy used in the manufacture and transport of a product), the less concrete and steel you have in your system, the lower your carbon footprint. The less electricity you use, the lower your ongoing impact will be on a day to day basis.

Durability is also important because if you have to replace components, then the long term carbon footprint increases. Zero discharge willow facilities also have a carbon footprint both in construction and for the pump (needed to get a good spread of effluent across the full length of the system).

However, if you harvest the willow and use it as a biomass fuel to offset oil or electricity, then your carbon footprint will be negative. Over 20 years, a willow facility will soak up almost as much atmospheric carbon as a standard mechanical treatment system will emit in electricity used. This is only of climate value if the willow biomass generated is used to offset heating oil within the house.

With this in mind, it may be worth considering the future of our energy supplies. Climate science suggests that we need to get our atmospheric carbon dioxide levels down from the current levels of 400ppm to at least 350ppm in order to make human life on this planet tenable.

If we select sewage treatment systems that can work well without any electricity inputs, then this will ensure that our sewage continues to receive good treatment even if electricity is rationed or is taxed to the point where running costs become too high. Human ingenuity is such that we can certainly find solutions, but given the current lack of easy alternatives to electricity generation then surely the prudent measure is to select one of the many systems that do not need it at all, or at least – as in the case of willows – that pay back more in biomass fuel than is consumed in electricity.

An overview of systems and costs

TypeCostIncluding installationElectricity cost per year (family of five)Septic tank desludging***
Septic tank with percolation€1,000-3,000/£750-2,200YesNil if gravity fed€150-300/£110-220
Mechanical unitAeration unit €2,000-9,000/£1,500-6,600 + percolationNo€70-275/£51-202€150-300/£110-220
Pump-fed filter media unit€3,500-5,000/£2,600/3,750No€20/£15****€150-300/£110-220
Constructed wetlands and reed beds*€1,000-4,000/£750-3,000 + cost of septic tank and percolationYesNil if gravity fed€150-300/£110-220
Zero discharge willow facilities*€15,000-20,000/£11,000-15,000 + septic tank and pump sump in case of new buildsYes€20/£15*****€150-300/£110-220
Waterless toilets**€50-2,000/£37-1,500No--
Urine diversion toilet€700/£515No--
Waterless urinal€175-300/£130-220No--
Faecal separator€700/£515No--
Faecal separator with specialist compost chamber components€6,000/£4,500No--

* All costs will vary according to site conditions but this is especially the case with planted treatment systems.
** The cost is not necessarily a function of quality or effectiveness. There is a large range of types, and it is important that this area is investigated fully before deciding upon a particular option. They are the most eco-friendly system available due to having no water use, and recycling all biomass and nutrients back to the soil safely and effectively.
***The frequency of desludging will depend on the amount of chemicals used in the home, and on the size of the population served and the tank size.
**** Electricity is only used when the liquid volume from the house activates the pump to the filter bed. Also factor in the cost of media filter replacement.
***** Additional costs include annual willow coppicing and spreading-line flushing, but you can do this yourself. If you use a wood-chip boiler or a log stove you can dry the willows and recoup some or all of these costs in fuel value.

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Feidhlim Harty

About Feidhlim Harty

Féidhlim is the director of FH Wetland Systems, an environmental consultancy business specialising in the design and planting of constructed wetlands, gravel reed beds and zero discharge willow facilities. FH Wetland Systems Ltd., 30 Woodlawn, Lahinch Rd, Ennis, Co Clare, tel. 065 6797355,

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