Wastewater management

These installations demonstrate a tendency to concentrate on the visually impressive or better known technologies. This can be to the detriment of less visual, unknown or misunderstood technologies which may have been more suitable.

The obvious risk is that installations will not perform to the required ecological criteria. This is bad for clients, the industry and the environment.

Some knowledge of the wide range of solutions is clearly desirable. A better understanding of the relative merits and drawbacks of various systems is helpful, along with a method to determine the most appropriate technology under a given set of circumstances.

Available technologies

Of the many sewage treatment systems available, almost all depend on physical settlement to remove gross matter, followed by a biological treatment stage using aerobic microorganisms.

Some systems trickle the wastewater over a media on which the organisms live surrounded by air. Others bubble air into the wastewater and encourage the organisms to form suspended flocks.

With either approach the dead organisms, having performed their function, are removed or recirculated as sludge. A clear effluent is left, with most of the original organic matter transformed into carbon dioxide and soluble minerals.

For smaller developments on appropriate sites, the septic tank is a natural treatment which should be considered as a contender for best practice1. This method physically removes solids, while correctly constructed leachfield or soakaway provides biological treatment, filtration and adsorption. A well designed and installed system can achieve near potable effluent quality when wastewater passes through just one metre of soil1,2.

With trickling filters, settled sewage is distributed over a bed of stone clinker or plastics media. A fall of 1·5 to 2 m is required, but pumping the waste on flat sites uses less energy than an equivalent powered system. These are robust and easy to maintain but require more civil works and capital cost than modern package plants.

Rotating biological contactors contain a slowly rotating cylinder of plastics disks, partially immersed in a tank of wastewater. Microorganisms grow on the disks' surface and are alternately exposed to air and sewage. There is no need for pumps or compressors. The units are modules and package plants for smaller projects.

The term 'reed beds' is used to describe a wide range of technologies2,3 from natural wetlands to highly engineered systems. Various reed bed technologies have proved to be highly effective in appropriate situations but there is no reason to consider them a panacea.

Vertical flow reed beds used in conjunction with horizontal versions can produce high quality effluent with no power input. However the site must have a suitable fall and the client be willing to take responsibility for routine maintenance.

The Living Machine was invented by Dr John Todd of Living Technologies. They typically use a large anaerobic stage, wetlands, aquaculture and floating macrophytes, combined with mechanical aeration in a greenhouse sited system.

The concept is sold as a green technology, but comparisons with conventional technologies show it can be out-performed.

Categorising the choices

Rather than categorise technologies into green and conventional, which can be misleading, more specific groupings are proposed:

• radical solutions;

• extensive systems;

• extensive with energy input;

• passive intensive;

• intensive systems;

• hybrids.

Radical technologies aim to avoid the problem rather than give an end-of-line solution. These are usually easiest to apply at the design stage. They include dry toilets, grey water irrigation and reuse, and stormwater source control.

Extensive systems are large and robust and have zero or low energy consumption. The system used depends on the soil type, available falls and area. Loading rates are limited by the oxygen transfer between atmosphere and wastewater, and retention time. The systems include wetlands and the sea.

Extensive systems with energy input include aerated ponds and living machines.

Passive intensive systems include trickling or sand filters, and vertical flow reed beds. The systems are often evolved from extensive systems, offering a smaller footprint at the cost of engineering complications. They use a vertical flow regime and gravity to improve the availability of oxygen.

Intensive systems typically reduce system size by using mechanical aeration. Some use chemicals to achieve oxidation, precipitation or nutrient removal while others combine biological and chemical processes.

Hybrids are constructed of any combination of the preceding systems and typically have an intensive first stage followed by an extensive polishing and buffering stage such as a pond or wetland.

Selecting appropriate systems

No technology can be considered appropriate out of context with the application. The first step in considering treatment options is to develop a clear brief of the client and legal requirements. Information can then be gathered about the site.

Checklists can help avoid premature conclusions. A general design checklist can include site details such as soil type and permeability, access and proximity to habitation. Client details include items such as budget, maintenance and image. The legal side includes EA/SEPA consents and concerns, planning, Building and Water Regulations, land ownership, shared systems and performance guarantees.

The sums required to make meaningful comparisons between solutions are usually simple. Subtler considerations may be required at the detailed design stage when faced with, for example, chosing between ongoing energy use or embodied energy at construction.

Differences are usually large enough to justify a simplistic analysis and a one-off project does not warrant the intimate environmental impact assessment worthwhile for a mass-produced product.

Products and solutions can be compared by:

• energy requirements;

• sludge production and removal costs;

• embodied energy;

• effluent quality;

• capital costs;

• running costs;

• the match of reclaimed water and suitable uses;

• chemical requirements.

Another useful process involves highlighting and prioritising key factors, the ranking of which will reflect personal concerns as well as site and project specifics.

In practice, visual statement is usually at the top of the list. This can create a conflict of purpose that could be debated by the design team to find a creative resolution. The important factor is that issues are considered, agendas and assumptions are identified, and the design process is transparent.

Case study one: public house and restaurant, Worcestershire

The existing package plant was failing its consent. An adjacent field was available, but the soil was too heavy for a soakaway. The landlord had no interest in green solutions but wanted an economic and easy to understand system.

The solution derived was a horizontal reed bed following the existing system. The extensive nature of the reed bed evens out peaks throughout the day and has ensured discharge consent levels are reliably met.

In this case cost was ranked as the most relevant factor, with reducing the problem at source the least important. However the latter may have been elevated to the list top had kitchen grease been a problem.

Invisibility and visual statement fell in the middle of the list of factors but need not have been mutually exclusive, and a visual system was discreetly located on this site.

Case study two: Glencoe visitor centre

The initial proposal for this flagship project was a wetland system with an area of around 1200 m2. This was chosen as a visual statement, an established green technology and for its educational value as part of a proposed nature walk.

A site examination however, showed this would be difficult to fit into a landscape with little flat ground, and a large reed bed would have an adverse impact on the site. The scheme would have required 2000 tonnes of aggregate and the export of an equivalent quantity of soil. The survey also found the existing trickling filter and settlement tanks were in need of repair.

Consultation with the SEPA suggested that required discharge standards could be met by upgrading the existing system. However calculations suggested that effluent volumes would be too high for the settlement tank.

Further analysis suggested that effluent 2/4 litre dual flush wcs, timed spray taps and other measures could more than half effluent volumes, thus obviating the need for a new settlement tank. These measures would also save energy and reduce the load on the trickling filter.

Greywater reuse was rejected due to a poor match between greywater volume and wc requirements, and the need for chemical additives with available systems.

When all the factors were prioritised, an apparently conventional technology proved the cheapest and most environmentally benign solution: effluent recycling.

Case study three: Felin Hescwm residence and holiday homes

The main issue for this site was access, as the property is located down a steep, narrow track. This made on-site sludge treatment a priority.

The owners initially trialed a Dowmus system, which replaces the septic tank with a free draining worm bed. However, despite various modifications and generous sizing, this Australian technology could not cope with the full hydraulic load of domestic sewage in the cool UK climate.

The technology has evolved to include a self-cleaning separator operating on the Coanda effect. This diverts most of the water past the compost bed. The liquid effluent is treated and dispersed in a swale area created with live willow and woodchip.

This technique is considered particularly appropriate for sites with poor access for heavy materials and a steep slope unsuitable for conventional soakaways.

The aerobic separator effluent is effectively odourless, an important consideration as the system is adjacent to a footpath.

Essential truths

Designers must ignore the superficial aspects of a technology and check for site and user compatibility as well as actual performance and ecological impact before choosing a system.

A detailed knowledge of biology or sewage treatment is not required in order to ask the right questions of manufacturers and designers, but a broad understanding of the options will help.