Low-impact sewage treatment: introduction

What is low-impact sewage treatment?

It’s any kind of treatment of sewage that attempts to minimise pollution and the use of water, energy and synthetic chemicals, and to turn our waste into something useful (fertiliser / soil improver).

Sewage treatment is the process of settlement and aeration used to reduce (sometimes to a great extent) pollution of nearby rivers or coastal waters by our sewage effluent. This typically involves the addition of energy and chemicals and the generation of sludges (of low value or even a toxic nature).


Pollution pathways: effluent from septic tanks can cause pollution to groundwater resources and surface water habitats if not adequately filtered through deep unsaturated soil. Source: Permaculture Guide to Reed Beds.

Sewage is the term used to describe domestic and municipal effluent (sewerage, by contrast, is the infrastructure itself – the pipework, sewers etc.). Sewage is typically comprised of ‘black water’ (from toilets) and ‘grey water’ (from sinks, wash-basins, showers, baths, washing machines etc.). Industrial effluents may also be included within municipal sewage. In a ‘combined sewer’ the runoff from roads, yards and roof surfaces (or stormwater) is also connected to the foul sewers. Note that connection of stormwater to domestic septic tanks is generally prohibited or at least discouraged due to the excess loading on the treatment system during times of heavy rainfall.

In the context of designing and creating low-impact homes and lifestyles, sewage is a self-limiting term because it is by definition the combination of potentially useful elements into one gloriously unpleasant mess. Thus I will also mention dry toilets and source separation as part of the overall suite of solutions we can adopt.


Newly-planted gravel reed bed system serving a single dwelling. Source: FH Wetland Systems.

So how do we find a way to move beyond our current modus operandi and treat sewage as a potential resource to be recycled instead of a waste to be disposed of? Generally when recycling, it is important to keep the different waste streams as separate and as clean as possible. This is no different with domestic wastewater, which has many useful constituent elements. Thus in addition to black water, grey water and stormwater, there are some additional terms that are gaining increasing use within wastewater engineering and design, as follows:

  • Yellow water: urine – with or without small volumes of water used for urinal or urine-separation toilet flushing.
  • Brown water: flush water leaving a faecal separation system such as an Aquatron unit.
  • Humanure: contents of dry toilets or faecal separation systems.
  • Humanure compost: compost made from humanure – ideally a clean, nutrient-rich humic material.
  • SUDS: Sustainable Urban Drainage Systems (or SuDS – Sustainable Drainage Systems) are filtration and/or attenuation systems used as part of urban drainage or even for one-off roof surfaces, yards or car parks. They help balance the runoff volumes from rainfall events and filter the water prior to discharge back into streams or groundwater.

Collecting ‘night soil’ (aka human faeces) used to be a common practice in many parts of the world. Collections were made at night (and still are in some places), and deposited on agricultural land. We wouldn’t recommend this without composting first, as some human pathogens can survive outside the human body long enough to be passed on via food (although the risk is low).

What are the benefits of low-impact sewage treatment?

We’ve learned to our cost that we can’t ignore water quality and still expect to have a healthy countryside and healthy lives. Thus, in the context of low-impact living, we need to diversify how we address the important issue of sanitation and to replace or back up the systems that rely on a ready supply of cheap fossil fuels for our electricity generation. proper sewage treatment is vital to the health of our wider environment.

One of the challenges with our current sewage treatment infrastructure is that it is heavily reliant upon fossil fuel inputs. The importance of removing sewage from our homes and treating it properly in on-site or municipal treatment systems can be easily overlooked until problems arise. For most households, power cuts are more likely to cause TV withdrawal distress than pollution anxiety. Yet without ongoing power supply to run blowers and pumps, our municipal sewage infrastructure will fail to work, and can discharge untreated sewage into our groundwater, rivers and seas. Many of the low-impact alternatives to conventional sewage treatment are more resilient to electricity shortages since they rely on gravity to move effluent through the system and to function. Examples include technologies such as reed beds, constructed wetlands, zero-discharge willow systems and willow filters as well as a range of source-separation technologies such as urine diversion toilets, faecal separator units and dry toilets. More details on these below.


Danish zero-discharge willow system. Note the two different heights of willow growth (photograph taken in late June). The foreground shows vigorous fresh growth after coppicing the previous February. The back half of the system shows growth in its third year, due to be coppiced the following February. Source: FH Wetland Systems.

Availability of clean water is one of the essential requirements for a healthy life. Directly related to this is the need for effective sanitation to keep our local groundwater and surface waters clean and healthy. We rely on a clean aquatic environment for drinking water supply, washing and cleaning, industry, agriculture, tourism and recreation. It’s also home to plants and animals in streams, rivers, lakes, ponds, wetlands, fens, bogs, estuaries and other coastal waters. These habitats in turn provide many and various ecosystem services such as food supply, water filtration and storage, flood prevention or amelioration etc.

Low-impact systems such as reed beds and constructed wetlands remove the need for electricity inputs to get the effluent clean, and thus achieve good effluent quality with a lower carbon footprint.

Waterless urinals are an example of low-impact sewage treatment

Waterless urinals don’t use mains water to flush, and can divert urine to be diluted (or composted with straw, sawdust etc.) and used as fertiliser.

Willow systems are excellent for taking up phosphates and nitrates, thus reducing the need for chemical dosing of effluent for nutrient removal. Willows also absorb atmospheric carbon as they grow and can be used as a carbon-neutral fuel to offset heating oil purchases. These systems also provide habitat for flora and fauna and enhance biodiversity.

What can I do?

There are quite a number of low-impact sewage treatment or sanitation options available, although they pertain mostly to people with a bit of space for an on-site system. In urban areas, you’ll be restricted to mains sewers or possibly a compost toilet if you have enough space in your garden for utilising the rich soil that is the natural result. However, simply peeing on your compost heap (at night perhaps, for modesty) is an excellent way to recycle N, P and K back into your soil and keep it out of the local municipal treatment system. For readers not connected to a mains sewer there are more options, with a variety of price tags and carbon footprint budgets, as listed in the image below.


Sewage treatment and sanitation options. Source: derived from Septic Tank Options and Alternatives.

Septic tanks and percolation areas are the most common on-site system, and where good depths of unsaturated soil exists for filtering the effluent, these can be low-cost, low-carbon and effective. Don’t forget maintenance though, to keep the whole system running smoothly. Annual desludging is typically recommended, or at least checking the sludge depth annually and emptying as necessary.

Mechanical aeration units may be used where soils are inadequate for providing good filtration, but these typically require ongoing electricity inputs, which isn’t great from an ecological perspective, and the annual running costs for electricity alone can mount up. Media filter units using peat, rock wool or coconut fibre also need to be pump fed, so are still reliant on electricity supply, but they are considerably more energy-efficient than aeration units which need power 24/7. Also, the three media types listed above are either stripped from natural peatland habitats, require high-energy inputs or need to be shipped around the world to get here, which isn’t exactly low-impact either.


Compost toilets require no water to flush, no pipework to sewage treatment plants, no chemicals to treat waste, and produce useful compost.

By contrast, reed bed systems are more eco-friendly since they can function effectively without any electricity inputs, where falls exist for gravity to do the work. Thus they have the advantage of being resilient to power shortages and having a zero-carbon footprint in their operation.

Willow systems are similar insofar as they are planted sewage treatment systems. They have the further advantage that as the willows grow they mop up atmospheric carbon, so they can be carbon negative over their lifetime, helping to provide fuel for the house as well as treating sewage. Zero-discharge willow systems can also have no discharge at all if they are carefully designed and built to evaporate all effluent entering them.


50% of phosphorus (P), 75% of potassium (K) and 90% of nitrogen leaves the human body as urine rather than faeces – which makes human pee a great fertiliser. Source: Michigan Tech.

Dry toilets are the archetypal low-impact sanitation option. Properly designed and built compost toilets have no discharge to groundwater or surface water, use no water for flushing, sequester carbon naturally as compost, and return biomass and nutrients to the earth where they belong. Dry toilets are a form of source separation.

Other source separation technologies include urine-diverting toilets, urinals (if the urine is collected for use as a fertiliser – waterless urinals are particularly effective for this since the storage volume requirements are greatly reduced), faecal filter systems such as woodchip filters or faecal separators. These systems can combine the ease and familiarity of flush toilet infrastructure with the safe and effective recycling of biomass and nutrients to agriculture.


Aquatron separator unit for faecal separation and composting. Note the rotating carousel chamber beneath the separator unit for ease of management and maintenance. The pipe at the top left corner is a grey water pipe, routed around the separator unit. Source: FH Wetland Systems.

It’s difficult to say which system is best, since every site will have different characteristics and every person will have different priorities and preferences. What this introduction demonstrates is that there are many ways to protect our local environment from sewage pollution and gain additional benefits such as biomass and nutrient cycling. We can replace or augment our existing infrastructure with a more eco-friendly method in our own homes and hopefully this will see a gradual shift in society to more sustainable systems for general use.

Thanks to Feidhlim Harty of FH Wetland Systems for information and pics.


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Féidhlim Harty is an environmental consultant and writer, and director of FH Wetland Systems Ltd., a company specialising in wetland and reed bed design, willow systems and habitat enhancement. He is the author of Septic Tank Options & Alternatives and Permaculture Guide to Reed Beds, both published by Permanent Publications.

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