Locals call them honey-suckers, but don't be fooled by the name. They cruise through the hi-tech streets of India's newest megacity, sucking up its lowest-tech problem: sewage. These trucks empty Bangalore's million septic tanks and pit latrines, where the majority of its 10 million inhabitants relieve themselves.
In other cities, sewage trucks discharge their cargo into streams and lakes, adding to local pollution. But in Bangalore, the honey-suckers head for farms outside the city, where their stinking loads are in demand to fertilise vegetables and coconut and banana trees. The farmers pay good money for human waste; it produces bumper crops.
The honey-suckers of Bangalore are evidence that the world of excreta is being turned upside down. Realisation is growing that our faeces and urine are not simply waste to be disposed of as fast as possible, but a valuable resource. Flushing sewage into rivers is not just an environmental catastrophe, it is also a ludicrous waste of nutrients that could be helping to feed the world.
Consider what you excrete. You produce some 500 litres of urine and 50kg of faeces a year. Besides water and organic carbon, your annual output contains about 10kg of nitrogen, phosphorus and potassium compounds, the three main nutrients plants need to grow - and, helpfully, in roughly the right proportions. This is sufficient to fertilise plants that would produce more than 200kg of cereals, says Christine Werner of German development agency GIZ.
Scale that up and the world's population excretes 70 million tonnes of nutrients annually. Applied to fields, this could replace almost 40 per cent of the 176 million tonnes of nutrients in chemical fertilisers used by the world's farmers in 2011.
Spreading human sewage on fields that grow crops doesn't sound appealing, but it is safer than you might think. Urine is normally free from the pathogens that cause diseases while soil helps to filter and clean bacteria found in faeces. Processed and handled correctly, the organic carbon and nutrients in urine and faeces will make soil more fertile and better able to hold moisture. The benefits would be huge. Recycling our waste onto fields would increase food output and make life a lot easier for poor farmers, who often cannot afford fertiliser. For example, a typical family in Niger, one of the world's poorest countries, annually excretes nutrients equivalent to 100kg of chemical fertiliser, worth a quarter of a typical rural income, according to a study by Linus Dagerskog, of the Stockholm Environment Institute in Sweden.
Replacing chemical fertilisers would also conserve supplies of phosphate minerals, which are running low. And while nitrogen in the atmosphere may be practically inexhaustible, converting it into fertiliser is a major user of the world's energy. Just as the world has to find ways to reuse scarce metals, so we need to find ways to recycle nutrients.
Most people in urban areas - an estimated two billion people - now have access to private or communal toilets. Unless they are connected to a sewer, these toilets empty either into pit latrines - usually little more than a hole in the ground that allows liquids to seep away while solids accumulate - or into septic tanks, where bacteria and an anaerobic environment encourage the solid waste to decompose.
These repositories need periodic emptying or they overflow into the streets. Few municipal authorities step up to the task, so private enterprise has swept in to fill the gap. Latrine and septic-tank emptying is a vast industry, little discussed and little regulated.
In India, despite laws banning the practice, an estimated one million people, mostly women and girls from lower castes, are still paid to scrape poo from the nation's 100 million or more tanks and latrines, usually with nothing more than a shovel and bucket. They dump the contents in nearby drains or on waste ground. In the Ghanian capital, Accra, most of the contents of the city's septic tanks end up on the ironically named Lavender Hill.
The fast-growing cities of the developing world are trying to deal with their waste in the way most industrialised countries do - by connecting every building to sewer networks. These take sewage to distant treatment plants that remove solids and other dangerous contaminants before discharging the effluent into rivers. But the infrastructure needed is vast and expensive, and the treatment is energy-intensive, according to Stanley Grant, professor of the University of California, Irvine, in the United States. It also leaves behind solids, which contain valuable nutrients, that end up as landfill.
Sewer networks also rely on huge amounts of water to flush toilets - water that in many places could be better used for drinking or irrigation. Dealing with the waste from flushing toilets typically requires more than a third of a city's water supplies, which means growing cities taking water from farmers who need it to irrigate crops and feed growing populations.
As a result, few of the world's megacities - and even fewer of the thousands of medium-sized urban areas - have fully functioning sewer networks. And of those, only about 10 per cent deliver their contents to functioning sewage treatment works. Most discharge raw waste into rivers, where it turns thousands of kilometres of waterways into lifeless open sewers. Further downstream, raw sewage helps create dead zones that now cover 250,000 square kilometres of ocean.
"We need to take the waste out of waste water," says Grant.
He and others are urging governments to take a fresh look at what we are trying to achieve with our sanitation systems. They should be based not on flushing our problem away but on "closing the loop" in our nutrient cycle, says Pay Drechsel, of the International Water Management Institute (IWMI), in Colombo, Sri Lanka.
Drechsel says it's a good thing that farmers in some parts of the world are recycling sewage onto their fields - even if they are doing it unofficially, usually clandestinely and often outside the law.
They are reviving an old tradition - and one that has continued in parts of China. Before the invention a century ago of the chemical process for converting nitrogen from the air into the nitrates plants can use, sewage was widely spread onto urban "sewage farms". Traditionally, it was collected in the dead of night to avoid offending people's sensibilities - hence the term "night soil" - and used to grow vegetables and other crops.
Campaigns to improve public health and the introduction of flush toilets meant the practice grew obsolete in most places. (Anecdotal evidence suggests night soil was still being used in Hong Kong's New Territories up to the 1980s.) Even so, where sewer systems were developed, farmers still sometimes competed for the network's outpourings. In a few places, this has persisted.
"China's use of night soil … is probably the reason that its soils are still healthy after four millennia of intensive agriculture," wrote author Rose George, in a 2008 edition of Slate magazine; and since the 1890s, most of the sewage from Mexico City has been piped untreated to the fields of the Tula Valley, to the north. Today, that megacity's 21 million people continue to fertilise more than 100,000 hectares with their faeces. The remains of the city's digested beans, tortillas and chilli peppers double yields of corn and almost triple the rentable value of farms, says Blanca Jimenez, of the Mexican Academy of Sciences. In essence, poo has made Tula Valley farmers wealthy.
The practice is also going through a purple patch in many urban areas in the developing world - especially in dry regions where farmers value the guaranteed year-round irrigation as much as the nutrient supply. In Pakistan, sewage grows a quarter of the country's vegetables. In the Indian state of Gujarat, farmers compete for sewage at annual auctions, preferring it to freshwater irrigation.
Now, the honey-sucker trucks in Bangalore are offering farmers another option - the sewage from millions of septic tanks and pit latrines. Increasingly, the drivers of these trucks have found they do not have to run the gauntlet of public opprobrium by dumping their loads onto wasteland or into drainage canals. Farmers within and around cities will gladly take their "honey".
"Sometimes the drivers charge the farmers, and sometimes they pay them. It depends on the season and the market," says Vishwanath Srikantaiah, of Biome Systems, a Bangalore-based consultancy that has investigated the practice in the city.
Typically, farmers put the sewage into drying pits to kill pathogens and to concentrate the nutrients so they can be dug into the soil more easily. During the dry season, however, they pour still-liquid sewage into dug channels, like regular irrigation.
The economics are good. Like the outflows of sewers, latrine slops increase the income of some farmers by thousands of dollars. Meanwhile, a single truck driver can service a population of 20,000 people and generate an income of US$50,000 a year, twice the price of a new truck.
Vishwanath says septic tanks emptied by honey-suckers offer not only a cheap alternative to the construction of sewers but a superior solution - saving water while delivering fertiliser to farmers, improving soils and boosting food production. Their services should be scaled up, not shut down.
Not everyone agrees. The biggest argument against agricultural recycling of sewage - whether from sewers or latrines and septic tanks - is that it carries disease. While urine is largely pathogen-free, faeces are rich in viruses, bacteria and worms. There are more than two million deaths a year worldwide from diarrhoea and other diseases associated with human waste. Most of these are down to poor hygiene, such as a lack of hand-washing, and are concentrated in areas where people still defecate in the open. Farming or eating crops fertilised by sewage is thought to play a minor role.
The trouble is, however, that there has been little reliable research conducted. A rare study of farmers, by Indian researchers, looked at 22 villages near the Musi river, which is little more than a sewer for the city of Hyderabad. It found that almost half of households irrigating their fields with the sewage flow had reported fever, headaches and skin and stomach problems during the previous year - twice the rate in a village that used clean water for irrigation. The highest disease rates were among women who weeded the fields.
Another study looked at what happened to the crops grown by sewage farmers in the cities of Ghana. Most of them grow salad vegetables such as lettuce that are sold in street food and eaten by some 700,000 people, says Drechsel. He calculates this could cause up to half a million cases of mild diarrhoea a year, nearly one per consumer.
The instant reaction is to ban the practice. But a more practical approach would be to improve hygiene. To maximise the benefits of recycling sewage onto land without creating health problems, safe practices for handling faeces are vital, Drechsel adds. The parasitic protozoa and viruses present in faeces cannot multiply outside the human body, so simply storing the waste in ponds before applying it to the fields kills many dangerous pathogens as the sewage dries out. But this requires months rather than weeks to be effective. Things can be speeded up by sprinkling wood ash or rice husks over the faeces, or by adding other alkaline materials such as lime. In combination with washing salad vegetables before sale, this can eliminate more than 90 per cent of the health risks, says Dennis Wichelns, principal economist at IWMI. Incinerating the waste destroys all pathogens and parasites, but it reduces the nutrient content. The problem, Wichelns admits, will be finding ways to encourage farmers and food sellers to adopt such practices.
The best way to grab most of the advantages of nutrient and water recycling without creating health hazards is to treat sewage before giving it to farmers. A typical sewage works will remove obvious solids such as sanitary towels and then leave the rest to settle at the bottom of ponds, before using bacteria to eat some of the organic material. These processes can remove most pathogens while leaving behind most of the nutrients.
Irrigating with treated sewage effluent is increasingly popular in developed countries short of water, too. For example, Israel uses about 70 per cent of the treated effluent from its sewage treatment works for irrigation.
With more intense chemical treatment, sewage effluent can be reused as drinking water. In Singapore, for example, they have branded their treated effluent NEWater.
"It is cleaner than regular tap water," says Yap Kheng Guan, senior director of Singapore's water utility.
While most of the NEWater goes to industries that need pure water, such as microchip manufacturing and pharmaceuticals, some is added to the city's drinking water reservoirs.
Orange County, in California, filters treated sewage through rocks beneath the ground, before pumping it up to fill the taps of more than two million residents. And London's drinking water has typically been drunk several times by people living in towns upstream of the River Thames, each time being cleaned up and returned to the river before being extracted again.
The truth is that the days of "flush and forget" must come to an end, even in the developed world. We should be recycling our faeces and urine in the same way we recycle scarce metals. In some places, that will involve advanced technology. But in much of the world that is a long way off. And where water is in short supply, even flushed sewer systems may be an unaffordable luxury. For billions of people in developing countries the best option, both economically and ecologically, may be septic tanks, honey-suckers and the return of the sewage farm.
At our disposal
In Hong Kong, the Drainage Services Department is responsible for collecting, treating and disposing of waste water.
According to the department, about 93 per cent of the city's population are served by the public sewerage system, which includes a pipe network of about 1,600 kilometres in length, about 200 pumping stations and 69 treatment works collecting and processing about 2.8 million cubic metres of sewage per day from residential, commercial and industrial premises, prior to disposal at sea.
The department is, however, running limited reclaimed-water schemes at 15 sewage-treatment works, the two most extensive being at the Ngong Ping and the Sha Tin facilities, on Lantau and in the New Territories, respectively.
The first tertiary treatment works in Hong Kong to produce reclaimed water, in 2006, Ngong Ping Sewage Treatment Works uses a three-tiered process to reduce organic pollutants, suspended solids, nutrients and pathogenic organisms in sewage. The water produced is odourless and safe, says the department. It is used for flushing in nearby public toilets and those in the Ngong Ping Cable Car Terminal, and also for rearing aquarium fish and controlled irrigation within the treatment works.
The facilities in Sha Tin can produce about 1,000 cubic metres of reclaimed water every day, mainly for use in the irrigation of plants there and the dilution of chemicals required for the sewage-treatment processes.
Says the department: "It is expected that valuable information will be collected on how reclaimed water can be used in other parts of Hong Kong."