Wastewater recycling concept for an urban multi-storey building

Transcription

1 Wastewater recycling concept for an urban multi-storey building Thilo Herrmann & Thomas Hesse ith Ingenieurbüro für technische Hydrologie, Am Kirchensteig 9, D Schobüll Tel (0) Fax epost: Stadtwerke Hannover AG, Dept. 1122, POB 5747 D Hannover Tel (0) Fax , epost: Keywords: Nutrients, potassium, sanitation, urban drainage, vacuum toilets Abstract: An existing building comprising of 32 flats built in 1962 was renovated and equipped with vacuum toilets with 1,4 litres per flush. The effluent of the toilets is collected separately by a vacuum pipe system. The energy and water consumption was determined. The flushing water consumption decreased from 40 l/p/d to 9 l/p/d. An innovative system for blackwater recycling is described. INTRODUCTION Travelling around in Germany south of Hannover the shape of bleak barren hills emerge on the horizon: The residues of ancient and recent potassium mining, fig. 1. A main component is kitchen salt, easily washed out by rainwater, fig. 2. Even decades after the mines have been given up, the heaps are a continuous source of salt polluting surface and groundwater (Spiegel 1998). Figur 1 The chloride concentration in the river Werra may increase to 9 g/lit in a dry summer, sometimes leading to fish kill (HAZ 1997). The reason for potassium mining is mainly the demand of agriculture for fertiliser and the chemical industry. In the 19 th century and before the faeces and the waste was brought back from the cities to the farmland. By the widespread introduction of the gravity sewer in the 20 th century the faeces have been diverted from the arable land to the rivers. Modern sewage

2 treatment plants eliminate wastewater nitrogen to a high extent. But sewage sludge contains only a little portion of the original nutrient content of the faeces. Most of the energy demand of treatment works is used for aeration, needed to transform ammonia to gaseous air nitrogen. On the other hand the fertiliser factories use energy to transform air nitrogen to ammonia for the production of fertiliser. This is a cycle, but may be not an energy efficient and sustainable one. For potassium (K) the balance is quite different: The elimination efficiency of advanced wastewater treatment is less than 4 % (Rodewyk 1979). The potassium passes the treatment works unaffected. Assuming advanced treatment, the recovering of phosphorus by sewage sludge seems to be efficient: About 90 %. But recent research results show that the plant availability of the phosphorus in the sludge is only around 25 % of the total phosphorus content. Due to the necessary dosing of P-precipitation agents over the theoretical reaction equilibrium it may be that the application of sewage sludge is decreasing the plant availability of the phosphorus in the soil. Thus, according to the German EPA the use of sewage sludge in agriculture is not justified (UBA 2000). Facing the problems of contaminated sewage sludge and the pollution of German rivers by effluents of sewage treatment plants and combined sewer overflows the idea came up to collect the toilet effluent (blackwater) and the rest of the household water (greywater) separately to reuse the blackwater after treatment as fertiliser in agriculture. The introduction of vacuum toilets Up to now the use of vacuum toilets is known from ships, aircrafts and modern trains. Some hospitals provide vacuum toilets to collect the excreta of people treated by radioactive substances to store it for radioactive decay. Vacuum toilets are so far applied under conditions, where there are special requirements for the transport or the necessity of storage of the toilets effluent. In 1991 it was the first time, when 12 private flats in Norderstedt near Hamburg have been equipped by vacuum toilets. The objective there was to investigate the saving of flushing water. Project design Since the elimination of organic carbon and nitrogen is not desired, the blackwater should be treated anaerobic. Vacuum toilets need about 1.4 litres per flush. This allows the collection of the excreta in a concentrated form. After addition of organic household waste to the blackwater, the anaerobic treatment is feasible and biogas is gained. To realize the first step of this sanitation concept, a four-storey housing building with 32 flats was retrofitted with vacuum toilets and a separate blackwater and greywater pipe system. The building was constructed in 1962 and a complete retrofit was necessary in Most of the apartments are rented out under welfare status. The building was insulated, the windows and the plumbing and heating installation were renewed. A block heat-power unit driven by natural gas produces electricity and heat for the building. Peak demands of heat are supplied by a gas boiler, the peak demand and the surplus of electricity is drawn by or delivered to the public net. The residents remained living in the building during the Figur 3 Vacuum toilet in the flats constructions. In April 1998 the vacuum toilets have been set in operation. Fig. 3 shows a vacuum toilet in a flat, fig. 4 the building after renovation. Table 1 gives some data of the building.

3 Figur 4 The project building equipped with vacuum toilets Table 1 Parameters of the project building Ground area 671 m² 60,37 m x 11,12 m Housing area 1900 m² Number of toilets toilet per flat Number of residents 73 to 80 average 78,4 Fluctuation 9 changes of tenants during 40 months The building is situated in a densely populated district of Hannover, built between 1960 and 1970, where some other innovative sanitation systems have been installed during the last 8 years. Under initiative of the local water supplier, the Stadtwerke Hannover AG and the authors of this paper there have been built various different systems for greywater treatment and reuse, urine separating toilets, a willow plantation for waste water evaporation, a soil filter for rainwater reuse and infiltration. The area is called OekoTechnikParkHannover and you can visit all the installations under regular operation. Under http.// you will find an illustrated description of all energy and water projects in this district. Vacuum system Figur 5 shows the scheme of the vacuum system. The blackwater pipes are kept under subatmospheric pressure by a vacuum station. The vacuum station consists of a buffer tank, vacuum and wastewater pumps and an electronic control unit.

4 vacuum pipes d = 50 mm closing valves electronic control unit used air outlet over the roof 33 vacuum toilets 1,2 lit/flush wastewater outlet 2 wastewater pumps 2 vacuum pumps vacuum buffer tank 0,6 m³ Figur 5 Scheme of the vacuum black water system The toilets are connected via the vacuum pipes with the buffer tank. Within this tank air and water are separated. The pressure is hold steadily between 450 and 550 hpa by running two vacuum pumps. The wastewater pumps empty the tank when a defined level of water is reached. The water level is detected by electrodes, diving at an angle of 45 into the tank. Excessive air is exhausted over the roof of the building. Up to now the blackwater is diverted in the existing public sewer. The anaerobic treatment of the blackwater will be realized later on an experimental scale. The functioning of the vacuum transport is demonstrated in Figur 6. A B C A Prior to flushing: The valve is closed, about 500 hpa below atmospheric pressure in the pipe system. B Triggering of the flush: The valve opens, wastewater and 70 l atmospheric air are sucked in. C After the flush: The valve closes, the sucked air is expanding and drives the wastewater plug through the pipe. Figur 6 Scheme of the flushing

5 The toilet is opened and closed by a pneumatic valve. The air pressure to drive the valve is provided by the difference of the atmospheric pressure in the bathroom and the vacuum pipe. There is no electrical installation necessary at the toilet. Figur 7 gives the design of the total system. 32 flats 32 vacuum toilets 78 residents 1900 m² housing area Vacuum buffer tank Wastewater pumps Vacuum pumps Toilet demonstration for Figur 7 Design of the vacuum system Figur 8 Vacuum station in the basement of the project building; on the right: vacuum pumps, in the middle: buffer tank, on the left: wastewater pumps.

6 RESULTS Water consumption The water and energy consumption has been measured in the period from April 1999 to August Figur 9 gives the daily consumption of flushing water and electricity for the vacuum station. The daily flushing water consumption varied between 580 and 740 litres, the mean is 697 l/d. The specific consumption per person was 7.5 to 9.4 litres per day with a mean of 8.9 l/p/d. There is no explanation known for this variation. The number of inhabitants was determined according to the data of the residents registration office. In about every third flat the tenants changed during the evaluated period of 28 months. May be the flushing habits of different tenants were different. The water consumption for flushing toilets prior to the retrofit is not known. Assuming a flushing water demand of 40 l/p/d using conventional toilets, the daily consumption of flushing water would be 3140 litres for the building. Thus the saving by the vacuum toilets is calculated to 890 m³ per year or 11.4 m³ per person and year. The water price in Hannover is 3.21 /m³, for drinking water including wastewater charges. The yearly saving in water costs for the building is determined to 2,853 /y or /p/y. [lit/day] [kwh/day] possible energy consumption after optimization 1,75 kwh/day Jan. 98 Jul. 98 Jan. 99 Jul. 99 Jan. 00 Jul. 00 Jan. 01 Jul. 01 Jan. 02 flushing water consumption for the total building energy consumption of the toilet system Figur 9 Water and energy consumption of the vacuum toilets Energy consumption In contrast to conventional gravity toilets, where the energy for the transport through the pipes is provided by the potential energy of the mass of flushing water, the vacuum station is consuming electricity for the vacuum and wastewater pumps. The total energy consumption was measured to 2008 kwh per year or 26.6 kwh/p/y. The specific costs of public electricity are 0.18 /kwh what 0

7 means yearly costs of 361 for the building or 4.58 /p/y. Regarding the electricity consumption versus time in figur 9, there is a strongly decreasing trend. The reason is, that the vacuum pumps are running part of the time against closed valve to heat up. The heating up is necessary to evaporate the water out of the pump oil. The times the pumps are idling have been reduced several times, so the energy consumption decreased over time. Nevertheless there is still a potential to reduce the running time of the vacuum pumps by 80 %, when the engine oil would be heated up by an external heating system. This might be done by an oil-water heat exchanger run by hot water from the heating system of the building. The costs of this optimization will be calculated in the near future. Costs The investments for the complete vacuum system haven been about three times the costs of conventional toilets. The investment according to cost groups are shown in table 2. A quarter of the investments was financed by subsidies of the regional government. These subsidies are financed by revenues from groundwater abstraction taxes, the so called Wasserpfennig (water penny). Investments Price in Euro number incl. taxes Vacuum toilets 32 20, Vacuum pipes 1 10, Vacuum station, mounted 1 27, Divers 1 5, Installation 1 9, Planning 1 8, Sum of investments 82, Costs of conventional toilets incl. Pipes, estimated 32-28, Additional costs for the vacuum toilet system 53, Subsidies from the regional government, sponsored by the groundwater penny Sum of additional expenses for the housing company -29, , Various extraordinary costs: Scientific evaluation, technical consulting 1 3, Vacuum toilet for demonstration purposes 1 1, Sum extraordinary costs 5, Table 2 The annual capital costs and the current costs for maintenance and repair were published by Herrmann and Hesse (2002). Up to now it can be summarized that the saving in water costs can pay back the deduction, the interest for the capital and the costs of ordinary maintenance. Due to several blockings in the ground pipes, caused by misuse of the toilets as waste bin by the residents, the system up to now could not be operated cost efficiently. Although, after an technical optimization

8 of the system, we expect the system to run cost efficient. Details of these problems are described in Herrmann and Hesse (2002). DISCUSSION At a flushing water consumption of 9 litres per person and day the concentration of organic carbon in the blackwater can be calculated to 1,5 g/lit TOC, when the TOC of the urine is not considered (Calculations according to data from Geigy 1995 and Koppe/Stozek 1998). The organic carbon in urine derives mainly from urea and therefore does not contribute to the production of methane. For an economic production of biogas the concentration should be higher. The addition of organic and food waste may double the carbon content. These could be taken in the system via the toilet or bigger items via a public grinder, which is connected to the public vacuum sewer. A significant increase of the gas production could be achieved by the addition of used cooking fat. A separate collection in the households organized like the collection of hazardous waste or used batteries should be evaluated. The fat has to be inserted directly into the digester via a mixer, to prevent deposits in the pipes. This is a well proved method of agricultural biogas operators. The installed vacuum toilets demand 1.4 litres of water per flush according to manufacturers data. Meanwhile there are vacuum toilets available demanding only 0.3 litres per flush, but not yet in ceramic quality. The implementation of the suggested vacuum-based blackwater recycling system is more effective, when there is no public sewer already existing. The investments for vacuum sewers are lower than for gravity sewers. This cost benefit may compensate the additional costs in the private households. The rearrangement of the existing gravity sewer system will took a long period of time, since there is a lot of capital invested which has to be deducted during the lifetime of the system. The contamination of the blackwater with excreted drugs and their metabolites needs further research, since these substances have been detected in drinking water which was abstracted from bank filtrate. REFERENCES Geigy 1985: Wissenschaftliche Tabellen Geigy, Teilband Körperflüssigkeiten, 8. Aufl. 1977, 4. Nachdruck 1985, Ciba-Geigy AG Basel HAZ 1997: Zuviel Salz in der Werra, Hannoversche Allgemeine Zeitung Herrmann, Th. & Th. Hesse 2002: Vakuumtoiletten im Wohnungsbau, gwf Wasser Abwasser, April 2002, Oldenbourg Vlg. Koppe/Stozek 1998: Paul Koppe, und Alfred Stozek, Kommunales Abwasser, 4. Auflage, Vulkan- Verlag Essen Spiegel 1998: Der Spiegel 7/1998, S. 68 TWJ 1993: Trinkwasserjournal 1/93, S. 6 UBA 2000: UBA Jahresbericht 2000, Hrsg. Umweltbundesamt, S. 167 UBA 2001: Daten zur Umwelt 2000, Hrsg. Umweltbundesamt