Technical and Economic Analysis of Dual Water Networks in
Buildings
José Carlos Soares Esteves
Extended Abstract
Instituto Superior Técnico, Universidade Técnica de Lisboa
1. Introduction
Water scarcity is an event becoming increasingly common around the world, in which many
regions witness periods of drought where the supply does not meet demand, resulting in a
challenge of its uses and restrictions to its consumption, which affect the economic
development and the standard of living of its inhabitants.
The growth of economic activities and standard of living maintenance of populations are
therefore reliant on the improvement of the uses and control of water demands.
The understanding of concepts linked to the sustainable management of water resources
implies the adoption of new strategies including the search of alternative new sources of water.
It is thus necessary to satisfy the populations’ needs of water by developing sustainable and
economically viable solutions.
Human activities cause an increasing deterioration and degradation of water resources. The
growing demand of water is due mostly to the exponential growth of its consumption for various
purposes: agricultural, domestic and industrial.
The water consumption figures are taken as the measured volumes of water that are removed
from natural water sources, and are used for various different human activities or just end up
lost in the water supply system. Domestic consumption varies significantly with the region's
location and type of housing, consumption habits of its occupants, cultural aspects and climate.
2. Framing of the Problem
The average distribution of household water is carried out as follows: showers and baths 32%,
exterior uses 10%, losses 4%, toilet flushing 28%, washing machine 8%, dishwasher 2%, and
faucets around 16%, see Chart 1. [1]
- 1 - Average distribution of household water
Losses 4%
Exterior uses 10%
Shower and bathing 32%
Toilet flushing 28%
Washing machines 8%
Dishwashers 2%
Faucets 16%
Chart 1 - The Average Distribution of Household Water [1]
Based on the chart 1, one can say that the use of water on bathrooms are the most common,
being that toilet flushing, as well as the use of water for showers and baths, altogether account
for a weight of 60% of the total water consumption in housing. It should be pointed out the fact
that only a small fraction of domestic consumption is due to human consumption (drinking and
food preparation). It is concluded that most of the household water uses, around 38% of total
uses, does not require the use of potable water, corresponding to the sum of uses for toilet
flushing (28%) and exterior uses (10%). The use of appropriate technologies for developing these new sources of water supplying, is
today, along with the improved efficiency of use and control of water demand, a key factor to the
solution of water scarcity on a global level.
The reusing of water is an important strategy for sustainable management of water resources.
Reusing water means using wastewater for a particular purpose, leading such to a socioeconomical benefit to populations in general.
Treated wastewater can be used for urban consumption, whenever it is not necessary the use
of high quality water, being that all waters used should be without risk to public health. Of its
possible uses, the following stand out: toilet flushing, watering gardens and golf courses,
watering sports fields, air conditioning, washing of containers of municipal waste and street
cleaning. [14]
The use of treated wastewater (lower quality water) for secondary uses in water networks in
buildings thus constitutes an added value in socio-economic terms and an effective alternative
in saving and rationalization of water consumption.
- 2 - 3. Technical Analysis Dual water networks in buildings are characterized by having two distinct networks of water
supplying: one for drinking water, corresponding to the primary use, and one independent
network to supply non-potable water, associated with secondary or non-potable use.
The main factor that prevents regular use of dual network systems in buildings is the cost
associated with building a second network for transport of non-potable water made up of
sections of pipe with smaller diameter, which means a doubling of investment as the network of
drinking water is always necessary.
In the implementation of innovative projects, mechanisms should be established for awareness,
information and education, particularly through the distribution of brochures, articles, technical
support, documentation and specialized books that demonstrate the operation and maintenance
of building systems, being that these documents should be intended for general public purposes
and workers specialized in urban sanitation.
The goal is to make this study a technical and economical assessment of a water supply
system on a building, with proper sizing and design using criteria based on pre-existing
technical-hydraulic standards. The water networks being studied consist of traditional systems
supplied with safe water compared with the systems set up with an innovative component of
drinking and non-potable water (dual water systems).
In the analysis of this work, the urban consumption of non-potable water is intended only for
secondary uses (toilet flushing and take-off points intended for street cleaning). This type of
reused water (treated wastewater) comes from a wastewater treatment plant, where tertiary
treatment is carried out with well-defined stages.
The evaluation of a water supply system implies the analysis of technical and economic factors.
In this work, it’s intended to scale different types of water supply networks in buildings,
traditional and dual in various different cases, with alternation in the materials used in pipes, as
follows: galvanized steel (GS) and multi-layer composite pipe (ML). The size of the building also
varies with each case being studied and the number of stories above the ground level is: 5story, 10-story and 15-story buildings. The alternation between a dual system and a traditional
system, the material used in pipes and the size of the building, sums up to 12 different case
studies, see Table 1.
Material
Size
Traditional water supply system
Dual water supply system
5-story traditional / GS
5-story dual / GS
10-story traditional / GS
10-story dual / GS
15-story traditional / GS
15-story dual / GS
5-story traditional / ML
5-story dual / ML
10-story traditional / ML
10-story dual / ML
15-story traditional / ML
15-story dual / ML
Table 1 – Analysis of Case Studied
- 3 - Each case study of water supplying systems in buildings analyzed is the subject of sizing and
design: the primary water supply extension, individual water extensions, the distribution lines,
supply extensions and water pressuring booster system when necessary. The pipes for drinking
water were designed to supply cold water and hot water inside the residential units. For dual
systems it was necessary to add a new supply network of non-potable water.
The network of non-potable water (treated wastewater) on dual systems is intend to supply only
toilet flushing in residential units and take-off points for cleaning (ø15 mm) placed at the ground
floor level and in basements.
Water consumption in a fire-fighting system is difficult to quantify since the occurrence of a fire
is an event of probability and of random nature. The scope of the present study does not include
the presence of a water network for fire fighting as well, and may be a suggestion for inclusion
in future studies.
The simultaneous design of two separate and distinct water supply systems in a building come
up as a challenge to the designer, and, although it is not yet common practice, it is necessary to
apply existing legislation to regulate traditional water networks and adapt it in a sensible and
consistent way of sizing and designing of these new water supply systems.
It is not expected changes in the diameters of the pipes of the drinking water network in
residential units, from the traditional to the dual distribution of water. This is due to the small
reduction in flow caused by the low scaling value set for the minimum flow of use of toilet
flushing and take-off points for cleaning. In addition, the number of these devices is small
compared to the total number of devices for use in the water network being studied.
4. Economic Analysis In this work, different rates of drinking water were used, as presented on Table 2. For the
calculation of these figures some aspects have been taken into account, namely:
• The tariffs used in Portugal are usually of progressive type with two levels of
consumption. Tariffs per m3 are higher for the level of annual consumption of 200m3, and
lower for the level of annual consumption of 120m3;
• A tariff review and possible increase in the value of the water tariff paid by the consumer;
- 4 - Thus, Table 2 presents the values of drinking water tariffs that were used in this work.
3
Drinking water tariffs (€/m )
Case
3
3
Consumption level of 120 m /year
Consumption level of 200 m /year
1,000
1,100
1,200
1,300
1,400
1,100
1,200
1,300
1,400
1,500
A
B
C
D
E
Table 2 - Values of Drinking Water Tarrifs (€/m3)
The most appropriate method for the analysis of the proposals in this paper is the method of "Life Cycle Cost" or (LCC). The different proposals compete between them only in relation to
costs, not being provided any other data, income or benefits, which may interfere with the choice. For comparison of the alternatives for each specific area, the method of analysis by (LCC) is usually performed on the model of the present value (PV LCC), which corresponds to
the sum of the discounted costs of each of the periods being studied and calculated by the
equation (1), expressed in [23] thus: n
Ct
t
t = 0 (1 + d )
PVL CC = ∑
(1)
PVLCC = Present value of LCC;
Ct = Sum of all relevant costs, resulting from the selection of the alternative solution, occurring
in year t;
d = Discount rate;
As the method of economic evaluation applied (LCC) uses cost values, the alternatives that
minimize the overall cost are those that correspond to the lower values of PV LLC.
Based on the model of economic evaluation called method of "Life Cycle Cost", the calculation
of the rate of wastewater is done regardless of the different case studies of dual systems of
water supplying. The indifference to the rate of treated wastewater then translates the value that
"balances" the system, or, in other words, it is the maximum permissible value that the investor
considers paying in order to avoid losses due to the adoption of a dual system instead of a
traditional system of water distribution.
- 5 - 5. Analysis Results
Thus, in Table 3 is presented the fare of treated wastewater in the situation of indifference for
different case studies, using different rates of drinking water standards as previously mentioned.
Tariff of treated wastewater in the situation of indifference (€/m3) Tariff ty pe applied for drinking water Case study A B C D E 5‐story Trad./Dual ‐ GS 0,801 0,901 1,000 1,101 10‐story Trad./Dual ‐ GS 0,300 0,400 0,500 0,601 15‐story Trad./Dual ‐ GS 0,185 0,285 0,384 0,485 5‐story Trad./Dual ‐ ML 0,810 0,909 1,009 1,109 10‐story Trad./Dual ‐ ML 0,312 0,412 0,513 0,612 15‐story Trad./Dual ‐ ML 0,195 0,295 0,395 0,494 Table 3 - Value of Wastewater Tariffs on the Situation of Indifference
3
for Each Case Study (€/m )
1,200 0,700 0,585 1,209 0,712 0,595 The values in Table 3 allow to set up chart 2, which demonstrate the variation in the value of the
tariff of wastewater to the situation of indifference when compared with different values of
drinking water tariffs, combined with cases.
Situation of Indifference
Case Study (GS) Trad./ Dual
1,400
1,300
Treated Watewater Tariffs (€/m3)
1,200
1,100
1,000
0,900
0,800
5 Story
0,700
0,600
10 Story
0,500
15 Story
0,400
0,300
0,200
0,100
0,000
A
B
C
D
E
Drinking Water Tariffs
Chart 2 – Variation of the Value of Wastewater Tariffs for a Situation of Indifference
(case studies with GS piping)
- 6 - The values in Table 2 allow to set up chart 3, which demonstrate the variation in the value of the
tariff of wastewater to the situation of indifference when compared with different values of
drinking water tariffs, combined with case studies with Multi-Layer composite pipes – Geberit by
Mepla (ML). Situation of Indifference
Case Study (ML) Trad./ Dual
1,400
Treated Wastewater Rate (€/m3)
1,300
1,200
1,100
1,000
0,900
0,800
5 Story
0,700
10 Story
0,600
15 Story
0,500
0,400
0,300
0,200
0,100
0,000
A
B
C
D
E
Drinking Water Tariffs
Chart 3 - Variation of The value of Wastewater tariffs for a Situation of Indifference
(case studies with ML piping)
6. Conclusions
To develop this study, one predicts that the rate of treated wastewater, already updated to the
year 2011, has a value of around € 0.50 / m3. This value corresponds to the amount to be paid
by the final consumer, and already includes the associated costs and profit margins inherent to
the management entities.
We can conclude that for the different values of drinking water tariffs used, a dual water supply
system is always economically viable for the case studies of 5-story buildings, using either
galvanized steel or multi-layer of Geberit by Mepla.
We conclude that the economic viability of dual systems like these decreases with the
increasing number of floors in a building, due largely to the high costs associated with
purchasing, maintenance and the electrical components of the water pressuring devices that
are necessary for the proper functioning of the water networks.
- 7 - The water dual systems are better economically for buildings with few floors, and for buildings
with more floors is necessary to conduct a more accurate economic evaluation and better
regulate the function of costs over the lifetime of the study.
Finally, we conclude that for certain cases of the present study, the escalation of tariffs used
benefits the adoption of dual systems in buildings that allow the reduction of tariffs applied to the
consumers of water, as well as a decrease in the amount of water taken from the water
resources available thereby contributing to an efficient use of water.
- 8 - References
[1] Almeida, M. C.; Vieira, P.;Ribeiro, R. (2006). Uso Eficiente da Água no Sector Urbano –
Série Guias Técnicos nº8, ERSAR, Lisboa (Portugal).
[2] Canha da P. A.(1986). Instalações em Edifícios - Volume VI (Instituto Superior Técnico),
Lisboa.
[3] CELE da APDA (2004). Água e Saneamento em Portugal – O Mercado e os Preços.Lisboa
(Portugal), APDA.
[4] CELE da APDA (2006). Água e Saneamento em Portugal – O Mercado e os Preços. Lisboa
(Portugal), APDA.
[5] CELE da APDA (2008). Água e Saneamento em Portugal – O Mercado e os Preços. Lisboa
(Portugal), APDA.
[6] CESUR (2007). Relatório IRAR/ERSAR n.º 3/2007,Análise dos Tarifários de Serviços de
Gestão de Resíduos Urbanos em Portugal, Departamento de Estudos e Projectos, Lisboa
[7] Excreta and Greywater, (2006).Who Guidelines for the Safe Use of Wastewater.
[8] Geberit (2011). Tabela de Preços de Sistemas de Tubagens em Geberit de gama Mepla
2010/2011, Lisboa (Portugal).
[9] Grundfos (2010). Catalogo Técnico de Sistemas de Pressurização hydro Multi-S, presente
em,http://www.grundfos.pt/web/homept.nsf/ab34d734e50cd531c1256569004a2b74/ee9c482f00
0401e7c12573f00036df5b/$FILE/hydro_MultiS_low.pdf (consultada em 11 de Março de 2010),
Lisboa (Portugal).
[10] Grundfos (2011).Tabelas de Preços – Janeiro de 2011, Lisboa (Portugal).
[11] IRAR/ERSAR (2007). Recomendação n.º 02/2007 – Utilização de Águas Residuais
Tratadas. IRAR/ERSAR, Lisboa (Portugal).
[12] IRAR/ERSAR (2007). Relatório Anual do Sector de Águas e Resíduos em Portugal Volume 1. IRAR/ERSAR, Lisboa (Portugal).
[13] IRAR/ERSAR (2009). Recomendação n.º 01/2009 – Formação de Tarifários Aplicáveis aos
Utilizadores Finais dos Serviços Públicos De Abastecimento de Água para Consumo Humano
de Saneamento Públicos de Abastecimento de Água para Consumo Humano, de Saneamento
de Águas Residuais Urbanas e de Gestão de Resíduos Urbanos. IRAR/ERSAR, Lisboa
(Portugal).
[14] Levy, J. Q.(2006). Novas Fontes de Abastecimento de Água, Reutilização e
Dessalinização, Lição de Síntese.
[15] Metcalf & Eddy, Inc.(2003). Wastewater Engineering: treatment and reuse – 4ªEdição,
McGraw-Hill.
- 9 - [16] Monte, H.M.; Albuquerque A.(2010). Reutilização de Águas Residuais – Série Guias
Técnicos nº14, ERSAR, Lisboa (Portugal).
[17] Patrocínio, T.(edição)(2007). Ficha Técnica. Tratamento de Águas Residuais Urbanas
(nº23). Construlink.com
[18] PEAASAR II(2006). Plano Estratégico de Abastecimento de Água e de Saneamento de
Águas Residuais 2007 – 20013. Lisboa (Portugal), Ministério do Ambiente, do Ordenamento do
Território e do Desenvolvimento Regional.
[19] Pedroso, V. M. R.(2004). Manual dos sistemas de distribuição e drenagem de águas,
LNEC, Lisboa (Portugal).
[20] Programa Nacional Para o Uso Eficiente Da Água: Versão Preliminar, Estudo elaborado
pelo Laboratório Nacional de Engenharia Civil (LNEC) com apoio do Instituto Superior de
Agronomia (ISA). Lisboa (Portugal), (2001).
[21] Quintela, A. C. (1981). Hidráulica, Fundação Calouste Gulbenkian, Lisboa.
[22] Ribeiro, F. L.(2004). Matemática Financeira, Economia e Qualidade da Construção
Edifícios, (Instituto Superior Técnico), AEIST, Lisboa (Portugal).
[23] Ribeiro, F. L.(2004). Métodos e Critérios de Avaliação Para as Decisões da Construção,
Economia da Construção – Parte III, (Instituto Superior Técnico), AEIST, Lisboa (Portugal).
[24] Ruas, M.; Vital, R.; Ramísio P.;Nunes E.;Medeiros C.;Santos A.A.;Beltrão J.;Farinha
P.;Olival L. (2005). Manual de Engenharia Sistemas de Pressurização, Grundfos, Lisboa
(Portugal).
[25] S.L. Tang, P.T. Yue, Damien C.C. KU (2007). Engineering and Costs of Dual Water Supply
Systems. IWA Publishing.
[26] Sultubos (2011). Tabela de Preços de Sistemas de Tubagens em Aço Galvanizado liso
serie média, Lisboa (Portugal).
- 10 - Internet Sites
[27] Ambiente Online - http://www.ambienteonline.pt/, consultada a 18 de Outubro 2009.
[28] Corsan (SITEL) ‐ http://www. corsan.com.br, consultada a 11 de Novembro 2009.
[29] Direcção Geral de Energia e Geologia - http://www.dgge.pt/, consultada em 18 de Março
2010.
[30] Entidade Regulador dos Serviços de Águas e Resíduos (ERSAR) - http://www.ersar.pt/,
consultada a 25 de Janeiro 2010.
[31] Geberit Portugal - http://www.geberit.pt/, consultada a 19 de Janeiro 2011.
[32] Grundfos Portugal - http://www.grundfos.pt/, consultada a 19 de Agosto 2010.
[33] Marine Associates, Inc. - http://www. marinesigns.com/, consultada a 18 de Outubro 2010.
[34] Portal São Francisco - http://www.portalsaofrancisco.com.br/, consultada a 13 de
Janeiro 2010.
[35] Revista H20água - http://www.h2oagua.com.br/, consultada a 13 de Dezembro 2009.
Legal Sources
[36] Decreto Regulamentar nº23/95 de 23 de Agosto, Regulamento Geral de Sistemas Públicos
e Prediais de Distribuição de Água e de Drenagem de Águas Residuais.
[37] Decreto de Lei nº379/93, de 5 de Novembro.
[38] Decreto de Lei nº 152/97 de 19 de Junho.
[39] Decreto de Lei nº 236/98 de 1 Agosto.
[40] Decreto de Lei nº243/2001 de 5 de Setembro.
[41] Decreto de Lei nº266-A/2007 de 31 de Maio.
[42] Lei da Água - Lei nº 58/2005 de 29 de Dezembro de Agosto.
[43] IPQ (2005) – NP 4434 – Norma Portuguesa sobre reutilização de águas residuais urbanas
tratadas na rega. Instituto Português da Qualidade, Caparica (Portugal).
- 11 -