Municipal effluent: Waste stream or resource?

The improvements in desalination technologies over the past 20 years are reviewed showing a halving of power costs. However the improvements in treatment of secondary effluent have also improved significantly with the advent of large scale UF/MF systems.

Introduction

With rainfall patterns predicted to alter and water set to become increasingly scarce, alternative water resources must be found to satisfy the growing demand for water. This paper discusses some of the alternatives and concludes that water is too valuable to use only once. Just as many commodities such as paper and plastic bottles can be recycled so then can water. Unlike many other recycled products, recycled water can be more valuable after recovery than before.

Desalination and Effluent Treatment

Generally the first response when there is a water shortage is a clamour to develop new water sources. If there are no natural water sources available then desalination is becoming increasingly attractive as both capital and operating costs continue to fall.

There has been a substantial cost reduction in both capital and operating costs of desalination over the past 20 years. The major component of the operating cost is power and there have been significant advances in the system design of the desalination plant in addition to major improvements to the membrane and energy recovery devices. Two plants constructed by Biwater are compared in the table below.

Table 1 - Comparison of energy use per m³ of product for a 2,500m³/d seawater RO train

Efficiencies as low as 2.5kwh/m³ are achievable for larger single stage seawater RO plants. The energy requirement was taken as the total energy required and includes abstraction pumps and pre-treatment. Claims of 2kwh/m³ only consider the power required for desalination.

However from the table below it is seen that both the capital and operating costs are lower when treating secondary municipal effluent than the desalination of seawater.

Reverse osmosis is significantly more energy efficient than thermal desalination methods such as Multi Stage Flash (MSF) and the desalination plant is independent of the power station.

Table 2 - Costs of producing water from secondary effluent and seawater (using reverse osmosis) ¹

The above comparison was made with ultrafiltration (UF) followed by reverse osmosis (RO) for treatment of the secondary effluent and reverse osmosis (RO) for the seawater desalination.

Sommariva² gives alternative operating costs (as seen in Table 3). If production or operating costs are considered rather than life cycle costs, then the production costs of seawater desalination using reverse osmosis is twice that of treatment of secondary waste water. MSF is considerably more expensive than reverse osmosis.

Table 3 - Production costs of seawater desalination and waste water desalination.²

The technological improvements in the treatment of secondary effluent are perhaps even more dramatic than those in seawater desalination. Below is a comparison of two water re-use projects supplied by Biwater.

Table 4 - Comparison of Secondary effluent treatment plants.

There are three major reasons for the considerable reduction in production cost. Firstly, the previously used cellulose acetate membranes which achieved only 96% salt rejection have been replaced with thin film composite poly amide membranes with a salt rejection of over 99%. The standardised membrane flux of the poly amide membranes is approximately three times that of the cellulose acetate membrane resulting in the lower feed pressure.

Figure 1 - Pre-treatment at Jeddah Water Recovery Works

Secondly, there has been significant development of the anti-scalant chemicals required for reverse osmosis plant which permit operation with a brine Langelier Saturation Index (LSI) up to +2.5, whereas the anti-scalant used in the 1980s was sodium hexametaphosphate which only permitted a brine LSI of +1.0. Consequently there is no longer a requirement to soften the water and to dose acid. Finally, the extensive “traditional” pre-treatment can be replaced with micro filtration or ultrafiltration. Membranes are often considered to be barriers to bacteria and the double membrane approach should result in almost complete removal of bacteria and virus.

Table 5 - Typical bacteria and virus rejection. (Note Log 5 = 99.999% removal)

Whereas the use of UF and MF membranes has become the accepted pre-treatment for water reuse projects where there is an existing sewage treatment plant, it is expected that Membrane Bio Reactors (MBR) will be used when there is no existing sewage treatment plant. An MBR combines secondary and tertiary treatment for the sewage in a single stage and will produce an effluent suitable to feed directly to an RO plant. The use of MBRs has become accepted within the UK for municipal effluent treatment where high quality water, low sludge production and limited land usage is required. Sewage treatment is not as well established in MENA and the use of an MBR for sewage treatment would provide a cost effective solution to produce a high quality effluent.

Figure 2 - Water Reclamation Works Scottsdale, Arizona, USA

Table 6 - Membrane Bio Reactors installed by Biwater for municipal effluent treatment.

Figure 3 - Membrane Bio Reactor, Campbeltown WwTW, UK

Singapore Experience

Singapore is an island approximately 683 km2 with of over 4 million inhabitants. Singapore signed a water supply contract from Malaysia on independence from Malaysia in 1965. This agreement is due to expire in 2013 and the Singapore Authorities are quickly developing their water resources to be less reliant on Malaysia.

They have adopted a 4 taps concept:

Tap 1 Continued sourcing of water from Malaysia.
Tap 2 Seawater Desalination
Tap 3 Expansion of water collection and storage through the construction of a dam at the mouth of the Singapore River.
Tap 4 The construction of plants to treat secondary effluent to produce low salinity recovered water. The Singaporeans know this as NEWater.

The treatment method is either UF or MF followed by RO with a final stage of ultraviolet disinfection (UV). PUB, the Singapore utilities company trialled this process for many years and the results were evaluated by leading international bodies and it was proven that the final water quality was better than that required by all health standards that apply to potable water. PUB has built 5 NEWater plants to date with a total capacity of 439,000m³/d. The use of NEWater for wafer fabrication processes, non-potable applications in manufacturing processes as well as air-con cooling towers in commercial buildings would free large amount of potable water for other potable purposes. Some is passed to the reservoir where its mixed with “natural” water and treated in a conventional water treatment plant. The Singapore Government and PUB have been very successful in convincing the public about the safety of NEWater and public perception worries have been overcome. Biwater supplied the Reverse Osmosis System for the fifth and largest NEWater facility at the 228,000 m3/day Sembcorp Changi NEWater Plant.

Figure 4 – Reverse Osmosis Plant, Sembcorp Changi NEWater Plant, Singapore

California Experience

Over abstraction from the aquifer around Los Angeles had resulted in seawater ingress into the aquifer. Secondary effluent has been treated by RO for over 30 years and the product water has been injected between the sea and the fresh water aquifer to prevent further sea water ingress. The treatment plant at Water Factory 21, Orange County is widely accepted as the centre of excellence for research into effluent treatment. The City of Oxnard’s Groundwater Recovery Enhancement and Treatment (GREAT) Program is a comprehensive water supply project that is designed to improve water supply reliability, sustainability and water quality for the future, and reduce the reliance on additional imported water. The GREAT Program combines wastewater recycling and reuse, groundwater injection, storage and recovery, groundwater desalination, and restoration of local wetlands to provide additional water supply source to the Oxnard Plain. Biwater designed and built the 28,400 m3/day membrane plant for the GREAT program. A grant was awarded to the City for its efforts towards environmental stewardship with Biwater’s innovative system design, incorporating efficient energy recovery devices.

Conclusions

The water available from both desalination and other scarce fresh water sources are too valuable to use only once and discharge to the sea and a treatment of the waste water for irrigation must be developed. The additional benefit of treating the sewage is that the sludge produced can be treated to produce a soil conditioner that would bind the sand together to produce a fertile, water retaining soil that should boost agricultural production. Furthermore the use of the soil conditioner and the irrigation water can be used to beautify the cities and gardens.

The development of desalination plants and advanced sewage treatment plants with membrane separation technologies, such as MBRs will increase the availability of water for industry and agriculture and reduce the reliance on the precious fresh/ground water supplies and desalination.

References

1 P.Cote et al “MBR beats tertiary filtration for indirect re-use” IDA Congress 2003
2 C.Sommariva “What is the real value of desalinated water” EDS Conference 2003
3 S.Arasu et al “Desalination marks Singapore’s first PPP initiative” IDA Congress 2005
4 Lim Chiow Giap “NEWater – closing the water loop” IDA Congress 2005
5 S.Beardsley et al “The economics of reverse osmosis and ion exchange” WATERTECH EXPO’94
6 Thames Water Publication “Water scarcity, solutions, conservation”
7 EEA report No 9/2005 “Sustainable use and management of natural resources”