Wastewater

last updated: 05/98

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1. Introduction

In the area of wastewater treatment, more and more concern is being placed on sustainability. Even the most advanced traditional sanitation plants still lose many nutrients to the environment which could be re-used in agriculture. The process is also energy intensive. This report examines some of the problems with the traditional wastewater treatment system and examines some alternatives, both broadly, and in detail. Other reports which shuld be addressed to provide a broader picture of wastewater treatment are the reports on Environmental Research in Europe and on Constructed Wetlands .

2. Disadvantages with Industrialised Nations’ Sanitation

Figure 1, below, shows a typical current sanitation system, of the sort adopted by the majority of developed nations. Note the input of fertilizer from outside the local area and the treatment of all wastewater (greywater, blackwater, storm runoff) at a central wastewater treatment plant.

Figure 1- Traditional Wastewater Treatment in Industrial Nations
[after Otterpohl et al., 1997]

treatment


The system is one that is far from sustainable, and has the disadvantage of making a useful leisure resource (a local river in this case) unusable. Indeed, research in the Netherlands [Water Science and Technology v.35 no.10 pp213-220] suggests that to create a sustainable system, wastewater systems will need to become 20x more efficient than they currently are. The main disadvantages of current technology include:

Nutrient Losses
Even with efficient watstewater treatment systems lose 20% of Nitrogen, 5% of phosphorus, and 90% of potassium. These nutrients enter waterways at levels that can be suffucuent to fuel algal blooms which kill wildlife.

High energy demand
The treatment of wastewater uses a great deal of energy. Typical demand in an industrialised nation is 110 kW/person equivalent/year. In addition, the conversion of nitrogen to ammonium/ammonia for use of fertilizer uses a significant amount of energy.

High Pollution Loads
Conventionally processed sewage sludge has high pollution load. This, combined with the loss of potassium, makes it unsuitable for use as a fertilizer. Non-recycling of waste leads to loss of carbon from the soil layer.

Mobilization of Heavy Metals
The presence of sulphur and heavy metals (e.g Cd, Hg, Pb etc.) in the sewage system can lead to mobilization of the latter. This can result in the mobilized heavy metals being discharged into rivers and/or the sea, and entering the food chain (such as occurred at Minimata).

3. Towards Sustainability I - Improved Wastewater Treatment

[from Water. Sci. Tech. V.35 No.9 pp121-133]

Achieving a sustainable wastewater system requires successful use of two techniques - source reduction of pollution, and treatment systems which recycle the nutrients and energy available in waste products. Figure 2, shows a more sustainable system for wastewater treatment. This system will be tested in Germany, where it is being used on a new housing development in Flitenbreite (near Lübeck). The area (3.5 ha) will not be connected to the municipal sewage system, but will be equipped with vacuum toilets (the type already installed in aircraft). The concentrated waste will be treated anaerobically with co-treatment in decentralized/semi-centralized biogas plants. The generated biogas will be used to provide energy (including district heating), whilst the solid sewage sludge will be used as fertilizer. Greywater (and stormwater) are kept separate from the sewage system, and treated by infiltration alone.

Figure 2- More Sustainable Wastewater Treatment in Flitenbreite
[after Otterpohl et al., 1997]

treatment


Greywater generated in the area will be treated either by biofilms, or in constructed wetlands [see the report at http://www.fujita.com/fruk/Reports/Wetlands.html ]. The initial plan is to use vertically fed constructed wetlands, with an area of 2 m2/p.e.

Figure 3, below, shows the effect of using technology similar to that which will be included in Flitenbreite. In addition total enegy demand for wastewater treatment will change from 110kWh/p.e./yr to -43kWh/p.e./yr (i.e. net energy generation of 43kWh by the system).

Figure 3- Waste reduction by using new systems at Flintenbreite
[after Otterpohl et al., 1997]

graph

4. Towards Sustainability II - Waste Minimization

The techniques and systems discussed above do much to make waste treatment more efficient, especially for domestic wastewaters. For industrial wastewaters however, such end-of-pipe approaches (ie treating the waste after it has been generated) can only ever be part of the solution. To make industrial systems more sustainable, it is necessary to look at the processes which generate waste, and to minimize the amount of waste produced. There are a number of techniques by which this can be achieved:

Material Changes
Changes in raw materials can lead to waste reduction. By using a higher grade ore, or one from a different source, levels of unwanted by-products can be reduced. If a process uses a catalyst, the cost of more frequent changing of the catalyst may be outweighed by savings in waste production (through obtaining higher yields of the desired product).

Equipment Modification/Process Changes
Modifications of existing equipment, or installation of new equipment, can lead to more efficient production and less waste. Changes in processes can lead to new, lower energy production pathways and can also contribute to less waste.

Recovery or Recycling
Recovery or recycling is often an extremely efficient method of waste reduction. Generated waste (often in wastewater) is either reused directly (closed system), or by treatment to recover valuable components (eg metals). In some cases wastewater may contain a component which is of no use to the generating factory, but which is an important raw material in another industry. This has led to the concept of waste exchanges.

These developments are often resisted by the industries which see only the capital cost involved in making changes to current processes. However, in the majority of cases, changes can be made cheaply, and will often pay for themselves through savings in raw material use and/or discharge of pollution fees.

4.1 Waste Minimization example - Allied Signals (Hopewell, Virginia, USA)

[from Water Science and Technology, 1997, pp.203-213]

Allied Signal is a major USA-based chemical company. Its plant in Hopewell, Virginia manufactures caprolactam ( a raw material used in fibres and plastics). Waste generated included high levels of ammonium sulfate and TOC (total organic carbon). Ammonium sulfate was a by-product of the production processe, but was converted into granular form and sold to the fertilizer industry.

Rather than bring in waste-minimization experts, the company trained its own engineers to understand the principles of waste minimization and had brainstorming sessions to consider new tecniques. The plant relied on evaporative processes and it was found that much sulfate was being entrained in exhaust from these systems. The solution? A simple mesh pad on the exhaust stream (to capture entrained sulfate), a sprayer (to wash the entrained sulfate back into the evaporating tanks) and a pressure gauge (to measure pressure accross the pad and determine when sulfate needed to be washed back into the tanks). The modifications were very cheap to make, but the results proved spectacular (as shown in figure 4).

Figure 4 - Sulphate emissions from the Allied Signals Plant (Hopewell, Virginia)
[after Asnari, 1994]

sulfate

Similar improvements were made in TOC discharges. In that case, the cost of modification to the plant was zero. A simple change in operating conditions of a single purge valve was all that was required.

5. New Technology

Although simple process changes, and cheap modifications can be used to make dramatic changes in pollution loads, this does not mean that there is no need for new technology, and the solutions that new technology provides. Below are two new technologies in wastewater treatment.

5.1 - The Urine-separating toilet

One approach to making wastewater managemnent more sustainable is the urine-separating toilet, which is rapidly gaining popularity in the Scandanavian countries (particularly Sweden). The toilet has been installed in a number of Sweden’s “ecological villages” and is proving have beneficial effects in reducing water use, and efficient recycling of nutrients to agriculture.

The urine-searating toilet appears similar to a convential toilet, but the bowl is seperated into two sections by a central wall. The front bowl is intended for the collection of urine; the rear bowl for the construction of solid waste. Each bowl can be flushed independently.

Figure 5 - The urine separating toilet

toilet

The effects of the urine-separating toilet on water consumption are dramatic. At one eco-village in Sweden, total water used in toilets dropped from 195 litres/person/day (Swedish average) to just 98 litres/person/day. At a second eco-village, Understen, total water used in flushing the front (urine-collecting) bowl averaged just 0.4 litres/person/day. The 0.4 litres of flushwater contributed to a total urine-solution collection of 1.4 litres/person/day.

In both Swedish eco-villages, the separated urine is stored in tanks, for periods of up to eight months at a time. When the tanks are full (or fertilizer is required for crops) urine is removed, and sprayed onto fields. Urine is a particularly good source of ammonia/amonium. Urea in urine rapidly dissociates in ammonia/ammonium, which is present at concentrations of 3.635 g/l. In addition, urine is much much lower in contaminants (such as heavy metals) than commercial fertilizers. Figure 6 (below) compares levels of contaminants in urine (when used to give 100kg Nitrogen/hectare of agriculturall land)with allowable levels in Sweden for the year 2000 (limits which will be among the lowest in Europe.

Figure 6 - Urine as a fertilizer

graph




Another of advantage in using urine as a fertilizer, is that much of the urine is available to plants directly as ammonia/ammonium. This is not the case with mineral fertilizers.

The separation of urine from solid waste, allows the solid waste to treated usefully also. At Björsbyn eco-village in Swedne, the solid waste passes through a septic tank and into a sludge composter. Sludge is removed from the composter, dried, frozen and composted further. After this treatment, it is then suitable for use on fields as fertilizer and as a soil enricher.

5.2 “New Generation” Reed Bed Filters in France

[This section taken from the report on Constructed Wetlands]

Since the 1980s, French researchers have investigating the use of Reed Bed Filters (RBFs) to treat sewage from small communities. Currently, there are 15 RBF systems in France, each treating waste from populations from 100-250 people (p.e) each. The systems are capable of receiving raw sewage (usually screened to < 2cm). They typically comprise several Type A (primary treatment, 1.15 m2/p.e) and 2 Type B (secondary treatment, 1.05m2/p.e) filters. Variant systems sometimes use 1 Type C (horizontal) filter instead of a second Type B.



Figure 7 - New Generation Reed Beds




wetlands in france


Typically there are a number of Type A filters. Each is loaded with waste for 3 to 4 days, before being rested for 6 to 8 days. These rest periods are critical to the functioning of the system; they allow mineralisation of Total Suspended Solids and maintain aerobic conditions in the gravel and rhizomes.

After 15 months of operation a detailed examination of the Montromant plant was undertaken over 48 hours. During the period of testing, temperatures varied from -8.5°C to +6.5°C . The results of the experiment showed the system to be highly effective at improving water quality (see Table 2)

Table1 - Performance of New Generation Reed Bed Systems (conc. in mg/l)


  Total COD d COD BOD5 TSS TP P-PO4 TKN
Raw Sewage 495 190 215 225 8.5 6.4 42.8
Filter A outflow 92 70 0 18 5.8 5.3 19.6
Final Outflow 58 40 16 12 5.6 5.1 10.1
Removal (%) 87.5 80 92.5 94.5 40 28 76

The application of raw waste to the A-type filters was shown to result in sludge accumulation of 1.5 cm/year, however this accumulation has not been shown to inhibit breakdown of waste, even at sludge-heights of 15 cm. Nevertheless, accumulation of such sludge should be allowed for in design of tank height (if it is not to be removed manually at a later stage).

6. Conclusions

The field of wastewater treatment, and its sustainability is a large and complex one. In this report, spome of the key areas of interest have been identified and discussed. This discussion is at a broad level, and more technical detail is available about most aspects covered. Please feel free to contact me by email for more information.

 

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