drs. H.J. Gastkemper*
* Stichting RIONED (RIONED Foundation (Dutch centre of expertise for sewer systems)), Postbus 133, 6710 BC Ede, NL (email: firstname.lastname@example.org)
In a highly developed country such as the Netherlands, the sewerage would seem to be complete: 98% of the homes are connected to the sewer system and 100% of the collected waste water is purified, and within a couple of years all purification will comply with the European norm for the removal of nitrogen and phosphate. This waste water system is perfectly capable of complying with its two essential social requirements, i.e. the advancement of national health and the reduction of environmental pollution. The current system is, however, not up to the challenge of the 21st century i.e. the reduction in the waste of raw materials, water and energy. Essential changes in the means of collection, transport and purification of waste water may be, therefore, expected. The social, ecological, water management and technological drives that lead to changes in the waste water system will be discussed below.
sewerage, future, wastage, system choice
Why do the sewers in the west look the way they do: a big pipe through which toilet water and other waste water is flushed together with the rainwater from roofs and roads?
The waste water used to just run into the stream or canal and be washed away with the rain. Because of urbanisation and in order to counter the stench, these waterways were often closed in. Sewers were laid following the outbreak of cholera and dysentery in the big towns and cities. The current sewers are, therefore, in essence the 19th century answer to a 19th century core problem: public health.
The waste was conducted through the sewer to outside the town. This all went well until the growth in population and activity exceeded the natural purification capacity of open water and the sewer caused huge water pollution. This is why sewage treatment plants were laid, literally at the end of the pipe. This was the 20th century solution to the 20th century problem: pollution.
The 21st centurys problem is wastage: on a global scale, this is the depletion of both finite raw materials (metals, fossil energy) and the inefficient and excessive use of renewable raw materials (fish, wood, freshwater). To focus on waste water, there is wastage of drinking water, clean rainwater, nutrients and fossil energy.
The reduction of wastage comes above the requirements of pollution reduction and protecting health. Our civilisation keeps raising the stakes, which is, in fact, necessary to maintain and improve the current standards of living.
In this article I will discuss the stimuli for renewal from four lines of approach, i.e. ecology, water quantity, technology and society, for which I will discuss concrete innovations. In summary I will make various suggestions for the way in which we could evolve towards new waste water systems from the current situation.
From an ecological point of view, reduction of wastage amounts to a reduction in the water cycle. In water policy, this principle is reflected in the preferred order of approach to waste water: Keeping the water clean has top priority, followed by separating the waste water streams and the last option is the purification. The first and final steps have been worked on for a long time, partly through prevention, licensing and phosphate and nitrogen removal in the purification plant. The preferred order of: keeping the water clean, separation and purification, has only recently arisen by the addition of the middle step, the separation of the streams and the substances. Quite a substantial amount can be separated, in principle. I will discuss four of the possibilities.
1. The separation of water streams
This involves keeping the rainwater separate from the waste water. The waste water is purified in a purification plant before it returns to the natural cycle. Rainwater immediately sinks into the ground or flows into the surface water, sometimes by the intervention of a simple filter (specially laid or already in place like the verge of the motorway).
2. Re-use of nutrients
People produce manure just like animals. This manure is virtually valuable and belongs on the soil. In the current times of artificial fertiliser and live stock manure surplus we sometimes appear to lose sight of this. It is unfortunate that the sewage sludge is no longer allowed for enriching the agricultural land, since this organic matter contains too many heavy metals and other pollutants.
One alternative is to collect the mineral stream separately and not let it mix with the waste water. This would mean separate processing of urine. The urine comprises 85% of the nitrogen a human secretes and around 60% of the phosphate. This approach implies a totally different waste water concept. Not only is the purification totally different but so is the collection and transportation. Moreover, special separation toilets have even been developed by well-known manufacturers. A start could be made on installing this system in buildings with larger numbers of people such as offices and barracks. The collection system is more complicated in individual homes. It is important to realise that dealing with urine in a different way is not only a technical matter but also demands a change in behaviour: if there is no urinal then men would have to sit down to urinate.
3. Energy exploitation
Current waste water purification costs energy: air is pumped into the waste water and bacteria decompose the pollutants with the help of oxygen. The energy in the waste matter can, however, be used. Methane gas is created by fermentation and can be used for generating electricity or heat.
The waste water itself is also a little warm. This energy can, in principle, be recovered using heat exchangers.
4. Re-use of water
The purified water the effluent is currently discharged into the surface water. There are several possibilities for doing something better with this water which has been rendered reasonably clean after so much effort. The effluent can be passed on to industry as a raw material in order to serve as a source for processing water. This effluent can also be allowed to infiltrate into the soil and can be pumped up again to be processed into drinking water. With a little additional effort the effluent can even be used as raw material for the preparation of drinking water. This is not a big problem technically. The most important reason for not doing this is the psychological barrier people have about not wanting to have the feeling of drinking their own waste water again.
Finally, it is possible to radically purify the waste water by best technical means. Membrane technology has made this possible. The Netherlands is leading in the development and implementation of membrane reactors for industrial and domestic waste water. This is more expensive but does provide a water quality which can definitely be re-used.
The previous remarks regard innovation from an ecological point of view. The second drive for renewal is the management of the quantity of water. For sewers this does not involve the effects of the rising sea level or the increase in river volume, but rather the local more extreme showers and high groundwater tables. The sewer system must be able to cope with this. This means a keen design and proper dimensioning. But the sewerage assignment is extended much further. This means, for example, laying drainage, raising ground surface levels and taking steps to drain surface run off from roads. The idea has hit home that sewer systems do not only concern the quality of water, but also the management of the quantity of water in built up areas. In my opinion, therefore, drainage and sewerage are part of the same field of study. This functional relationship must be a guiding factor in the assignment of administrative responsibilities. Good drainage is necessary for the correct functioning of public space, roads and parks. Moreover, there is a close link between sewerage and road management both in the design of the public area as well as in the cost awareness. The control over the sewer systems in the Netherlands lies in the hands of local government who wish to keep this responsibility.
A third impulse to renewal is, logically, technological development. Future predictions regarding this are not difficult to make: technical developments always tend towards greater efficiency and better compliance with existing and new social needs. In the not too distant future I expect:
- faster and more reliable damage classification by inspection with image identification
- increased monitoring of hydraulic performance and waste emissions
- a definition of the remaining life span of pipes based on observation and statistic comparison;
- better insight into functioning and managment of separate sewer facilities.
Tension in the technological development occurs between the improvement of existing techniques and the introduction of new ones. A good example of this is the improvement in efficiency intended to counter the waste of water. The Netherlands has just had a traumatic experience with this: the introduction of domestic water (a quality less than drinking water) into homes for toilet flushing and for garden watering failed. In spite of all the technical precautions, this water got into the drinking water network and some people did get sick. When introducing separate waste water streams or the direct infiltration of rainwater into the soil, sewer system experts hammer on constantly about the systems needing to be sufficiently robust. They are right in this, but this will not be acknowledged as long as the advantages of new systems are assessed as more important than the risks. One example of this is that separate collection is inherently more vulnerable than mixed. Yet in the Netherlands we do want to decrease the wasting of rainwater. The assignment is, therefore, to create robust solutions within new systems. Due to a lack of experience with the new techniques, this is not always a simple task.
In the fourth place, social requirements function as a catalyst to innovations. This can happen in extremely varied ways. I will mention four aspects below.
The relationship between sewerage and environmental planning
Luxury homes and companies are being built up everywhere on waterfronts. One famous example is the Docklands area in London. This living on the water could only be developed after the rivers were cleaned up and they stopped stinking with the help of the sewer system and water purification. In this case, the sewer system was a precondition for environmental development.
On the other hand, scarcity of space can also act as a stimulus for new technology. This applies, for example, to membrane filtration which requires less space both for its own installation as well as indirectly because of a smaller circle of stench nuissance.
In the third place, the relationship between the sewer system and urban design is increasing: the provisions for removing rainwater separately are intertwined with other measures such as road layout, traffic safety, urban renewal and public parks. An additional advantage to this is that sewerage becomes literally and figuratively more visible for residents.
Advantages of scale or self-suppliance
Sometimes argueing takes place between the supporters of large scale solutions such as we know today, with extensive transport systems and big purification plants and the supporters of small scale solutions: on the spot purification and as short a water cycle as possible. This battle is not always productive since the same target can often be attained with either a large scale or a small scale solution. An example is: reclaiming energy can be realised by the fermentation of the black water in combination with organic waste at local level, but also by fermentation of the sewage sludge which is released in the waste-water purification plants. What will be the outcome of this debate? This is determined to a great extent by costs: which solution is the cheapest? But the powers that be will also try their utmost to keep and improve the current system. I believe that we can expect to see small scale solutions mainly in rural areas, where there are specific concentrations of people or companies and in experimental situations.
The struggle for attention and money
Dutch local governments spent 1,067 million in 2003 on the sewer system. This amount does not include the sewer systems on private property and the purification of waste water. For 16.2 million inhabitants this amounts to 66 per person per year or 0.18 per day. The sewerage sector continually has to remind us of the need for adequate waste water facilities and the scope of the facilities necessary for this.
The costs of the sewerage will inevitably rise due to the lack of reserves for replacement, separation of the dirty water from the rainwater, and flood protection measures. The extent of the additional costs can be limited if they are correctly co-ordinated with public road facilities management. If the sewer system is changed, the costs increase sharply if the life expectancy of the facilities is not taken into account.
Resident and user
Everybody is user of sewers and understands that waste water and rainwater have to go somewhere. But the majority of people have no further grasp of what happens to their toilet and shower water. This does not generally pose a problem and that is what matters to them. This is the central criterion on which innovations to the waste water system are assessed by the average person: in day to day use it must be absolutely problem-free. For the rest, any alternative approach must satisfy the criteria: safe, comfortable, manageable and user-friendly.
The division of duties between private persons and the authorities is also important: which responsibilities and costs are covered by the authorities and which ones do private persons have to pay? In the Netherlands the authorities tend to take responsibility themselves. They are worried that the public is not capable of managing its own facilities adequately. This tendency is also visible in the management of small-scale purification plants in rural areas and in the prevention of water problems in home and on private property.
What consequences can the mentioned developments mentioned have on the current sewerage systems? Some developments have already sometimes very carefully been put into action. Comments on this are somewhat speculative since the new targets are not yet urgent enough in a social sense to be pursued vigorously. Targets may also be conflicting (e.g. membrane filtration also supplies reusable water but requires additional energy). Referring to countries with a high grade sewerage system, I can see the following developments for each system:
Combined sewer system
The separation of rainwater will be a high flyer, but a large number of combined systems will remain for hydraulic, environmental or financial reasons. Separation can easily be combined with measures to influence the groundwater level. The pollution from overflow will decrease, but these facilities will remain indispensable during heavy showers.
In new housing development areas it is standard to lay systems whereby dirty water and rainwater remain separate. In existing developed areas experience is being gained in separating rainwater from other waste water. A point of interest here is that the rain water is radically separated without taking extreme precipitation into account. Emergency facilities will always be required, such as on-road storage or emergency spillways. A great deal of knowledge and experience still has to be developed regarding the life expectancy and working of separation facilities.
Alternative collection and purification systems
Actually, the future has already begun: a pilot membrane reactor for the purification of waste water has already been implemented in the Netherlands. Anaerobic fermentation and the separation of urine and faeces is already done in developing countries. The exciting question is whether or not this will lead to a fundamental adjustment of the western waste water systems. I expect that it will, sooner or later, since society does want better, more efficient solutions; solutions which are better suited to our unlimited, expanding requirements and solutions which are more efficient with raw materials, energy, money and human effort.