Vacuum toilets in multi-storey buildings
Vacuum toilet systems, in addition to being relatively quick and easy to install, can achieve enormous water savings, resulting in significant environmental and economic benefits.
As cities around the world expand there is an increasing burden on key infrastructure, including water supply systems and waste water treatment and disposal.
If less water is used then the volume of waste water that must be treated is also reduced, so vacuum sewerage systems can form an environmentally responsible solution.
Some five years ago construction of the major Frei Caneca Shopping Mall and Convention Centre, which included a vacuum sewerage system, was completed in the bustling Brazilian city of Sao Paulo.
World Plumbing Review decided to take a look at how the 6-storey project was conceived and developed and, in particular, the effectiveness of the EVAC vacuum sewerage system after five years of operation.
Sao Paulo-based consultancy Procion Engenharia undertook hydraulic and electrical systems design for the Frei Caneca project.
A partner in the firm, Luiz Costi, says that right from the start the project presented major challenges.
“Several difficulties had to be addressed, including limited access through narrow streets, and space restrictions during construction due to the building occupying the complete dimensions of the ‘L’ shaped property,” he said.
“The logistics of construction proved to be a major challenge for the project team, as did negotiations regarding numerous legislative requirements.
“Difficulties were also encountered by the commercial team in selling space in the new venture due to the depressed economic conditions in the country at the time, and competition from nearby shopping facilities.
“I will always remember my partner, Levon Sevzatian, coming into our office with the plans for the new job. He was excited but also conscious of the responsibility of the technical design group, telling me, ‘We must think as a team; we will not have space for mistakes or delays, or isolated ideas, we need consensus. It will be the only way to success.’”
“The project consists of 74,000m2 (almost 800,000ft2) of construction with five levels of parking and one level for a supermarket below ground level. Above that, the mall consists of three levels with 182 stores, including one level to accommodate six movie theatres, restaurants, and a theatre and television school. Then, level four is the convention centre, level five is the main theatre, and the top level is for the administration and a heliport.
“In early 2000 the group started to discuss the concept for the structure. Until then we did not have much experience with steel structure, even though Brazil is a major producer and exporter of steel.
“Most of our construction projects had involved concrete structures. However, after considering the reduced lead-time, design requirements, and production, transportation and mounting constraints, our destiny was sealed.
“Codema Metalic Structures produced the metal pieces 400kms (248 miles) from Sao Paulo, and the pieces were assembled within four months.
“The natural technical choice was the use of dry wall (plaster board) to simplify the mounting of hydraulic and electrical systems. This would also reduce assembly time, and the impact of weight on the structure and foundation.”
Keeping water costs down
“To encourage prospective tenants, the team was looking at ways to keep ongoing costs down. One option for achieving this related to the cost of water,” Mr Costi says.
“A vacuum sewerage system with over 200 toilets would use only 1.2L (1/3 gallon) of water per flush compared to 6-12L (1½ – 3 gallons) with conventional toilets, but the investment would be higher.
“Finally, we arrived at a mixed solution: to use a vacuum system for toilets, as well as a gravity sewerage system for other fixtures such as urinals and wash basins.
“This also allowed us to program the installation as an independent phase of the construction process, and resulted in the same cashflow situation as if we had installed a conventional sewerage system.
“The more expensive parts, such as the central vacuum unit and the actual toilets, could be purchased within the planned supply and cashflow schedule.
“Other key issues such as close supervision also had to be addressed. Because there are few examples of installation of EVAC systems in large commercial buildings, the installation was done by workers who did not have specific technical experience in this type of project.
“Also, we were not sure how the new vacuum toilets would be received by the public, and whether there would be problems with vandalism and the cost of maintenance.
“The investing group, the construction contractor, and Procion, had indeed agreed to proceed with what could have been regarded at the time as a high-risk solution.
“However, the main reason that we proceeded was that one parameter was assured – while the price of water in Brazil was very low compared with other countries, it was going to increase.”
Operation
Vacuum systems operate on the difference in air pressure between the atmospheric pressure in the toilet bowl and the lower pressure created by the vacuum in the collection pipe.
When the toilet is flushed suction is created which draws the sewage from the bowl into the pipe. Air pressure is used to transport the sewage, and water is only used for cleaning the bowl.
The vacuum sewerage system installed in the Sao Paulo project consists of two main sewerage stacks for concentrated male and female toilets. There are around 170 toilets in the Mall and Convention Centre and the remainder are installed in the theatre and administration areas.
By connecting toilets to the stacks, a toilet can be isolated for repairs or maintenance without affecting the normal operation of the others.
The central vacuum sewage pumping room has two completely independent systems with four pumps and a tank each, and the public electricity supply is backed up with a diesel generator in case of a power outage.
Successful payback
According to Luiz Costi, today the mall is a successful enterprise – this year the investors are planning a 20,000m2 (215,000ft2) expansion.
“After almost five years the system is operating effectively, the economic result is positive and public acceptance is good,” Mr Costi says.
“There were virtually no problems in assembling the system. Installation was quick and there was minimal interference with other systems within the dry wall. A major benefit of the vacuum system proved to be the ability to move sewage vertically up the system, thereby providing added installation flexibility.
“However, the most outstanding advantage of all is the payback. Given higher water prices, the system has been completely paid for on the basis of reduced water consumption.
Overcoming problems
“One problem which the maintenance technicians have needed to confront is the number of strange objects discharged by the public into the system, including underwear and plastic bags.
“This has caused intermittent pipe blockages, so a small rooter machine with seven metres of flexible cord is now used to clear pipes once obstruction sites have been identified by tapping the pipes. A series of raised head cleanout plugs has been installed to facilitate entry to the pipes.
“After a couple of years of operation the maintenance group detected some encrustation inside the pipes. Initially this was removed once a year by filling the pipes with a solution of phosphoric acid (H2PO4) and water for 24 hours and then draining them.
“However, this practice caused a lot of debris to accumulate in the central tank and a different method is now used. Once a week the pipes are filled from the top of the risers and left overnight. By the morning all the encrustation is gone and there is little debris in the central tank.
“The maintenance people also carry out monthly tests of the vacuum in the pipes at the central pumping unit. The pumps have to operate with 18-20in (45.7-50.8cm) of Hg and if this falls to less than 12in (30.48cm) the toilets will not operate.
“After one year of operation the central vacuum sewage pumping room on the fourth underground floor showed a very strong smell of urea due to an inefficient exhaust system. Therefore, a new ventilation system with a 40cm (15¾ in) pipe was installed to expel the foul air.
“The energy consumption of the central equipment is not significant – we are talking of 3.3amps in a 380 Volt three-phase system (per group of pumps and tank).
“In summary, the higher price for water in Brazil means that the choice of a vacuum sewerage system for toilets in public buildings is a very interesting option. However, a competent maintenance team is important as well as strong support from the equipment supplier (EVAC).
“Investors in the Sao Paulo Mall and Convention Centre project are benefiting to the extent of $50,000 per year in water savings after the payback.”
Vacuum benefits
Apart from significant environmental advantages in relation to reduced water consumption, vacuum toilet systems can provide a range of benefits to building designers and installation contractors.
Vacuum systems are generally quicker to design and install than traditional gravity systems.
Because vacuum system pipes have a smaller bore than those used for gravity sewerage systems, they can be routed around obstacles such as beams and ventilation systems, and this can mean increased design flexibility.
Also, vent stacks are not needed, which further enhances design flexibility because the location of toilets, urinals, hand basins and showers can vary from floor to floor. This flexibility can be particularly important in refurbishment and retrofit projects.
Significant cost savings can be achieved in not having to dig trenches for a gravity system and the vacuum pipes can be located in a variety of positions, including ceiling voids and floor spaces. The force of the vacuum allows sewage to be lifted from one level to another.
Toilet odors are reduced because air in the toilet cubicle is sucked into the toilet bowl and discharged with the sewage into the vacuum pipe.
Potential leakage problems due to pipe damage are also reduced because the amount of liquid in the system is relatively low and any leaks are drawn inwards rather than flowing out of a fracture.
Vacuum toilets, which are available in both wall and floor-mounted models, are quiet and the flush cycle is much faster than conventional gravity toilets. When the flush button is pushed special valves inside the toilet activate the vacuum and water to clean the bowl.
As Mr Costi says, vacuum sewerage systems can reduce the long-term risk of increasing water prices and the Sao Paulo project has demonstrated that maintenance costs can be kept well within expectations.