Hong Kong has built the world’s largest sewage-sludge-to-power facility, and leads the way on water resource management

On a clear October day, moments before Hong Kong’s historic 2012 cross-harbour swimming race began, a beaming 68-year-old Lao Chong told reporters he had long dreamed of this moment; of reliving the days when he took part in the race as a young man. The race had been shelved since 1978 because of raw sewage and other detritus in Hong Kong’s famous Victoria Harbour.

When the starting gun rang out after a 33-year hiatus, 1,000-odd swimmers young and old plunged eagerly into the harbour. But the sentiment among the crowds that day ranged from elation to trepidation.

Hong Kong’s local authorities boasted that the water quality had vastly improved. Environmental groups said otherwise.

The rapid deterioration of Victoria Harbour’s water quality had resulted from sewage being discharged directly into the harbour without sufficient treatment. Since 2001, however, Hong Kong’s government has been rolling out a two-phase Harbour Area Treatment Scheme (Hats), to collect and treat local sewage in a more efficient, effective and environmentally sustainable manner.

However, water quality tests conducted at the time by local pressure group Green Harbour Actions and the Open University of Hong Kong revealed levels of E coli (a pathogen bacteria) to be as high as the range 10,000-30,000 per 100ml in some areas of the harbour. This far exceeded the maximum E coli count of 610 recommended by Hong Kong’s Environmental Protection Department (EPD) for local swimming beaches.

Mired in muck

Every day, Hong Kong’s residents generate 2.6m cubic metres of raw sewage, according to the drainage services department – the equivalent of 1,400 Olympic-sized swimming pools. About three-quarters is currently treated under Hats; the de-watered sludge is sent to landfill, the effluent is chlorinated and discharged into the harbour.

The remaining 25%, notes local lawyer Miriam Lao, is still discharged untreated into the harbour – which accounts for the high E coli counts in some areas. EPD spokesman YF Chau says his department “has conducted regular inspections of misconnected sewers and will take appropriate enforcement actions”, adding that phase 2 of the Hats scheme, which is currently under way, should also partly resolve the issue.

Local authorities have been grappling with Hong Kong’s sewage problem since 1998 when it became evident that, unless action was taken, by 2015 the region would be inundated with sludge.

Japan burns roughly 90% of its sewage waste, while the US and EU spread, respectively, 60% and 38% of their sludge onto agricultural land. China spreads 45% of its sewage sludge onto agricultural land, uses 33% on its gardens, sends 34.4% to landfill and incinerates 3.4%.

Hong Kong, by contrast, sends the whole lot to landfill. Not only does the sludge threaten to destabilise the landfill sites built into the city’s steep hillsides; without an alternative, the sites would be maxed out within the decade, says Chau.

Waste-to-power

After exhaustive studies, Hong Kong’s EPD decided to build a utility-scale waste-to-power incinerator. The sludge will be directly burned (without further drying) to heat boilers, generating steam that will drive two 14MW turbines to generate electricity.

Hong Kong’s sludge is ideal for combustion. Its high chloride content is a result of the city’s use of seawater for toilet flushing. While this limits its reuse potential as compost or soil conditioner, the chloride gives the sludge a very high calorific value, making it ideal for incineration without having to use additional fuel.

The plant will generate enough electricity to power not only the sewage treatment facility, but also a seawater desalination plant that will produce about 600 cubic metres a day of desalinated water, of which 160 cubic metres will be potable.

There will be enough electricity left over to power an environmental education centre an and on-site leisure complex with indoor heated swimming pools. Any surplus electricity will be exported to the power grid. 

The facility is being built in Tuen Mun (in the western part of the New Territories), by a consortium comprising Veolia Water-Veolia Environmental Services (VW-VES owning 60%) and Leighton Asia (owning 40%). Due to come online in the second half of 2013, it will relieve 11 sewage waste treatment facilities (including the Hats) of 1,500 tonnes of sludge a day by 2014, reducing the sludge-to-landfill volume by up to 90%.

By 2020, it will have the capacity to burn 2,000 tonnes a day. The leftover incinerator waste (ash) will be disposed of at landfill. The process will remove the headache of landfill destabilisation and possible groundwater contamination.

Sludge incineration and heat recovery is widely practised in Europe – mainly in Germany, but also in Belgium, Holland and Austria. The difference here is that the sludge is first dried to either co-supply coal-fired power plants, or to produce process steam. Hong Kong’s Tuen Mun plant by contrast, with a capacity of 28MW, will be the world’s largest self-sustained sludge-fired waste-to-power plant.

Concerns have been raised over potentially nasty emissions from burning the sludge, but according to Vincent Deleu, project manager for VW-VES (HK), the flue gas treatment technology being deployed ensures emissions are compliant with European directives and the EPD’s “best practicable means”.

He adds that any sulphur dioxide emissions, (one of Hong Kong’s biggest headaches when it comes to air quality), will be neutralised by injecting sodium bicarbonate into the dry reactor.

Up in smoke

The Tuen Mun project is arguably a poster child for sustainable sewage management. The only drawback is that the process does not include phosphate recovery. As Dr Tim Evans, chairman of the Chartered Institution of Water and Environmental Management’s (CIWEM) wastewater management panel points out, global reserves of phosphate are dwindling rapidly.

Phosphate is an essential agricultural fertiliser that cannot be substituted; without it, life on this planet would be impossible. China is one of the biggest consumers of phosphate fertiliser and Evans argues that by incinerating Hong Kong’s sludge, a valuable opportunity for phosphate recovery has been missed.

From this perspective, it’s fortunate that waste-to-power is not the only option on the cards. Researchers from the Hong Kong University of Science and Technology, Delft Technical University and KWR Institute have leveraged Hong Kong’s unique seawater flushing system to drastically reduce the volume of sewage sludge and to recover phosphate.

Flush for phosphate

By using sulphate-reducing microbes that converting organic pollutants into carbon dioxide, the “sulphate reduction, autotrophic denitrification and nitrification integrated (SANI) process” could minimise sludge production rates by 90% (to below 200 tonnes per day), reducing treatment costs and space requirement for wastewater treatment by over 50%.

In addition, the team is working on a urine phosphorous recovery technology to include in the SANI process. Test results to date indicate that 95% of the phosphorus can be precipitated as a valuable phosphorus-based fertiliser, struvite. A two-year large-scale pilot begins in April 2013 at Hong Kong’s Sha Tin sewage treatment works.

The speed at which waste streams are ramping up in Hong Kong and elsewhere certainly calls for more than one solution. For the time being, these sludge treatment projects present tidy, efficient solution to a dirty, great problem. The next task at hand is to tackle illegal sewage connections and ensure that effluent treatment processes are sufficient to clean up Hong Kong’s coasts, allowing the so-called “fragrant harbour” to live up to its name.

Water smart

Hong Kong is the only city in the world using seawater for toilet flushing on a city scale. The system currently delivers savings of 740,000 cubic metres per day of freshwater. 

Wiser water supply

Hong Kong International Airport is the first organisation in the world to deploy the Hong Kong University of Science and Technology’s novel triple water system, that combines freshwater, seawater flushing and cooling, and water recycling systems for air-conditioning, kitchen and laundry.

So far it has generated yearly savings for the airport of 52% in freshwater demand, up to HK$20m (US$2.5m) in electricity bills, and 17,000 tonnes of carbon dioxide emissions – the equivalent of taking 5,500 cars off the road.