Valentina Lazarova, Vincent Sturny and Gaston Tong Sang
10.2 ROLE OF WATER QUALITY AND TREATMENT TECHNOLOGY FOR THE TRUST IN WATER REUSETHE TRUST IN WATER REUSE
The adequate choice of the treatment technology, the tertiary ultrafiltration which allows almost total disinfection and removal of suspended solids, is the first of keys factors that ensured the fast growth of recycled water demand. In addition, the ability to provide high-quality recycled water without any interruption provided the required trust in water reuse from large end users such as the luxury hotels.
Figure 10.1 View of the Island Bora Bora (French Polynesia): aerial view (a) and water villas of luxury hotels (b).
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Treatment train for water recycling
The ultrafiltration hollow fibbers submerged membranes (ZeeWeed 500) have been chosen to polish a part of the secondary effluent and implemented in the existing wastewater treatment plant of Povai (Figure 10.2). The ultrafiltration membranes have small pore size of 0.035 µm, which represents an effective physical barrier for all microorganisms and pathogens, including protozoa, cysts, bacteria and viruses. The initial treatment capacity was 300 m3/d and was extended to 500 m3/d in 2008.
Recycled water is stored in two covered reservoirs and pumped into the industrial (non-potable) water distribution network after chlorination in order to maintain 0.5 mg/L chlorine residual.
Water quality control and monitoring
Table 10.1 illustrates the wastewater quality (raw sewage, secondary effluent and recycled water) for the last three years.
Despite the high variations of raw sewage characteristics, tertiary ultrafiltration consistently produced an effluent with a very good quality, free of suspended solids and with a very low content of organic carbon (COD and BOD below the detection Figure 10.2 Schematic flow diagram of wastewater treatment and recycling plant and views of the construction of the dual distribution system and the new storage tank.
Table 10.1 Water quality characteristics of the water recycling facility in Bora Bora.
Parameter Raw sewage*
Secondary effluent Recycled water (UF permeate)
Measured* Consent Measured Guide value
COD mg/L 595 (270–837) 31 (21–65) 90 15 (4–34) 40
BOD5mg/L 349 (200–540) 7 (,5–22) 25 4 (1–6) 20
TSS mg/L 238 (125–275) 9.5 (4–19) 35 ,5 20
Ntotmg/L 47 (30–70) 8.3 (2–18) 20 7.3 (2–17) 20
Ptotmg/L 6.8 (4.1–8.1) 2.5 (1.0–5.8) – 1.9 (0.45–5.8) –
E.coli/100 mL not analysed 105–107 – not detected 0/100 mL
Streptococci/100 mL not analysed – – not detected 0/100 mL
*Average value and limit of variations (32 monthly composite samples, excludingE.coliandEnterococcithat were monitored monthly in grab samples as colony forming units).
Thekeystosuccessofwaterreuseintouristareas129
limits). During the 5-year operation of the submerged UF membranes, three cases of contamination of the permeate with fecal coliforms have been detected due to valve leakage and overflow of unfiltered secondary effluents (up to 200 cfu/100 mL).
Enterococci(56/100 mL) andClostridiumspores (2/20 mL), which, as a rule are not present in the membrane permeate, have been also detected during this plant failure. To improve the reliability of operation of the ultrafiltration, the treatment plant was upgraded to not allow any by-pass of unfiltered water or external contamination of the permeate. An on-line turbidity measurement was recommended as an efficient measure to control any valve leakages or breaks of membrane fibbers. An additional barrier for efficient disinfection and control of biofilm growth in the distribution system is ensured and maintained by a final chlorination after storage tanks.
Main challenges for operation
By applying the recommended maintenance and cleaning procedures, a good recovery of membrane permeability was observed, 202 to 247 L/h · m2bar at 20°C compared to the initial value of 250 L/h · m2bar. It is important to underline that despite some minor operational failures and a major damage of the WWTP in September 2005 due to tsunami with seawater intrusion and loss of activated sludge from the clarifier, an excellent membrane flux was consistently maintained at 24+2 L/h · m2(20°C).
Preventive measures against membrane fouling are the crucial factor avoiding excessive loss of membrane permeability (on-line cleaning using scouring air, backpulses, membrane relax, automated maintenance cleaning). The frequency of chemical recovery cleaning varied from 2 to 6 months depending on the wastewater quality. By safety, recovery cleaning was carried out before reaching the fixed minimal value of 100 L/h · m2· bar. Compared to other membrane systems, more frequent recovery cleaning was required, especially during winter periods.
10.3 WATER REUSE APPLICATIONS
The main water reuse application is for landscape irrigation, mostly for luxury hotels (Figure 10.3), but the production of high quality recycled water enabled the development of the following other urban uses:
Boat washing.
Filling the water reservoirs of all fire protection boats.
Supply of 20 hydrants for fire protection.
Cleaning and landscape irrigation in all water and wastewater plants, as well as in the 70 pumping stations.
Cleaning and industrial uses in the composting facility.
Washing of construction engines and preparation and tests of concrete at 3 to 7 building sites, depending on the year.
Municipal nursery for the production of flowers, shrubs and other plants.
Municipal workshops (construction equipment, school buses).
Municipal stadium Teriimaevarua and the associated sport facilities.
Environmental enhancement by filling waterfalls and ponds.
Figure 10.3 Views of the hotel landscape in Bora Bora irrigated with recycled water.
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Evolution of the volume of supplied recycled water
The adequate choice of the treatment technology and the ability to provide high-quality recycled water without any interruption were the two key factors that ensured the fast growth in recycled water demand (Figure 10.4). A twofold increase in the recycled water consumption was observed shortly after the start-up of the new recycling facility, with an average annual volume of about 70,000 m3/yr and a daily demand up to 600 m3/d. The number of end users after the start-up of the new membrane recycling facility tripled compared to that in 2004.
10.4 ECONOMICS OF WATER REUSE
Project funding and costs
The capital cost of the water reuse program was covered by the international, territorial and French funding. The main challenge of the municipality was to minimise the risk of failure of tertiary treatment at acceptable operation and maintenance costs below the cost of seawater desalination. For the first 3 years of operation of the membrane facility, the operating and maintenance cost was about 0.68E/m3, which is equivalent to 38% of the annualised life cost.
The operating costs of the recycling facility in Bora Bora include the operation and maintenance not only of the tertiary membrane treatment, but also of the distribution system, which is predominantly underwater (submerged in the lagoon).
The main expenses comprise fixed costs for labour, repairs and maintenance, membrane replacement and water quality monitoring, as well as variable costs for chemicals and energy consumption (Figure 10.5).
Figure 10.4 Evolution of the recycled water demand and the number of end users.
Figure 10.5 Distribution of operation costs (tertiary treatment and distribution network).
Thekeystosuccessofwaterreuseintouristareas131
The main component of operation costs is labour, accounting for 46% of total operation costs. Despite the high price of electricity in this isolated tourist island (0.18E/kWh), the contribution of energy costs is only 14%, which is relatively low compared to typical values of tertiary MF/RO treatments of 26–32%. Chemical costs for membrane cleaning and final chlorination contribute to another 12% of operation costs. A new important part of operation costs is repair and maintenance, which rises to 21%, including membrane replacement. It is important to underline that the cost of spare parts and scouring equipment is also higher because the remoteness of this island, and thus, the high transportation costs. Water quality monitoring remains relatively low, 4% of operation costs, but a great part of the expenses are included in the labour costs. The average energy consumption at nominal flow is estimated at 0.36 kWh/m3, but a twofold increase to 0.62 kWh/m3 is observed when hydraulic load drops to 50%. During operation with flow rates below 30%, the energy consumption raises to 1.0–1.6 kWh/m3due to the high fixed energy needs of membrane treatment.
Pricing strategy of recycled water
After dialogue with the stakeholders and local communities, the municipality of Bora Bora decided to implement a tariff policy in favour of the local population: a two-part tariff with an ascending rate structure (Table 10.2). The social signal of this decision was‘The less you use the less you pay’. The progressive rate with the increasing volume of recycled water supply is a reminder of the lack of water and the necessity to save the resource. At same time, for a given category of end-users, a declining rate structure is implemented to encourage the use of recycled water.
According to this new recycling water pricing, established since November 2005, recycled water charges vary from 30 to 100% of potable water rate, depending on user category (water demand). In addition to these consumption-based rates, a fixed annual charge of 187Eis required for each connection. For comparison, the previous recycling water charge for polished secondary effluent (maturation pond effluents are not allowed for spray irrigation and other urban uses) was fixed at 0.67E/m3 regardless of water consumption. It is important to emphasize here that the use of recycled water (industrial non-potable water) and the subscription to this service are strictly reserved for professional needs of the population.
With the increasing number of end-users, a simplified pricing of recycled water was under discussion to provide additional financial incentives for water reuse (Table 10.3). In fact, it was observed that during rainy periods, a strong decrease in the demand for recycled water occurs. In such a context, the simplified pricing of recycled water that is expected to be implemented in 2011 will favour water reuse. The discount rate of recycled water will be from 26 to 48% of the drinking water rates for large users with a monthly consumption above 75 m3.
Table 10.2 User fees for recycled water in Bora Bora.
Parameter Criteria First block Second block Third block
Volume for large users, m3/month .350 m3 ,550 550 to 800 .800
Recycled water charge,E/m3 2.35 2.18 1.65
Volume for medium users, m3/month ,350 m3 ,110 110 to 200 .200
Recycled water charge,E/m3 1.16 1.08 0.88
Volume for small users, m3/month ,30 m3 ,5 5 to 10 .10
Recycled water charge,E/m3 0.76 0.71 0.67
Table 10.3 New user fees for recycled water in Bora Bora under validation.
Category of end-users Fixed annual charge,E/yr
Volume of recycled water, m3/month 0–30 m3 31–75 m3 .75 m3
Small users DN 25 209 0.67E/m3 1.68E/m3 2.51E/m3
Midium users DN 32/40 293
Large users DN.40 419
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High drinking water rates for large consumers due to desalination (200 to 700% of the rate for small consumers, i.e. local population), as well as increasing water demand of luxury hotels, and in particular the requirement for landscape irrigation and other non-potable urban uses, are important factors for the acceptance of water reuse. Consequently, the main challenge of plant operation was to minimize risk of failure of tertiary treatment at acceptable operation and maintenance costs below the cost of seawater desalination.
Benefits of water recycling
Large users such as luxury hotels were the first to recognise the economic benefits of water reuse as the cost of high-quality recycled water is, in fact, 2.5 to 3 times less expensive than potable water. Consequently, recycled water demand increased, encouraged also by the declining rate.
Another economic benefit that can be easily estimated is the prevention of revenue loss of building and tourist companies.
In fact, because of severe drought, at the end of 2005 the use of drinking water for non-potable purposes was proscribed and four important building sites and their landscaping were interrupted because of the lack of water for concrete preparation and tests. It would require the construction of one desalination plant for 3 hotel extensions, each with 100 luxury suites of 100 m2 each. The economic damage that could be caused by the delay of construction has been estimated at E2 to 3 million, and without taking into account the potential loss of revenue during the peak tourist season that would have been over E50 million. The economic and social benefits of this new recycling scheme were also recognised by the local and regional authorities, as the start-up of the membrane facility enabled them to postpone the construction of an additional desalination plant and to limit water supply interruptions for local residents that were frequent and unavoidable during drought periods.
In addition to the increasing reliability of water supply, the local population underlined another very important environmental benefit of the reuse of well-treated wastewater: the safeguard of the lagoon and its biodiversity. The fragile lagoon system of Bora Bora can be rapidly deteriorated by human activities such as accidental discharge of wastewater and/or wastes.
For the local decision makers and elected officers, the most important benefit of water reuse was the protection of local natural freshwater resources, for example, saving of 10% of drinking water for domestic and potable uses. The protection of the lagoon was also a crucial benefit. Consequently, Bora Bora has become the only municipality of French Polynesia, which has been awarded the“Blue Flag of Europe”. This label is highly prized by foreign visitors, mostly from northern Europe, who are used to select their vacation based on environmental criteria.
10.5 HUMAN DIMENSION OF WATER REUSE
Generally, technical issues are ranked as less important in the surveys of public perception of water reuse by individual consumers. However, these issues are crucial for the trust and acceptance of water reuse by large end-users such as industries, farmers and tourist facilities, as demonstrated by the experience of Bora Bora. In this case, water quality compliance, availability and reliability of the supply of recycled water were very important factors for the acceptance to supplement potable water supply with recycled water in tourist resorts with a high risk for direct human contact. The success of the water recycling scheme in Bora Bora is almost the only water reuse project with sprinkler irrigation approved by French Health Authorities since the mid 2000s. An important specificity of the new French regulation on water reuse for irrigation, published in August 2010, is the existence of several restrictions on spray irrigation in urban areas and golf courses, despite requirements on water quality and minimal distances. Health concerns and the precautionary principle were the main factors for some opposition of regulatory bodies for the implementation of landscape irrigation with recycled water (Lazarova & Brissaud, 2007).
In addition to technical factors, other economic and environmental factors greatly influenced the Bora Bora community acceptance of water reuse.