Wastewater is an undervalued source of water, nutrients and other recoverable elements. Collecting, treating and reusing wastewater helps to alleviate potential water stress, protects public health, mitigates the costly impact of pollution to ecosystems, and provides alternative water resources for agriculture and industry.
Water pollution is an increasing problem across the world – contaminating natural water courses, damaging freshwater supplies, adversely affecting ecosystems and endangering the health of billions of people. It is estimated that more than 80 per cent of global wastewater is discharged into the aquatic ecosystem without being treated or re-used. Major sources of this discharge are agriculture, industrial production, and untreated domestic run-off.
Mismanagement of wastewater by industry alone, is considered to result in as much as 400 million tonnes of toxic sludge being introduced into water sources annually. The true extent of industrial pollution is however, difficult to measure as discharges are poorly monitored and seldom aggregated. Wastewater from industry tends to be a process by-product, with each sector producing a different combination of pollutants. The result is a hazardous cocktail of heavy metals, solvents, oil, acids, sulfates, phenols, cyanide and organic chemicals all being released into the environment.
Agriculture is by far the largest consumer of water worldwide, accounting for nearly 70 per cent of all withdrawals globally. Saving just a fraction of this through effective wastewater management could alleviate water stress in all areas of society. Wastewater pollution from agriculture can be significant, with nitrate from manure and fertiliser run-off for example, often being the most common chemical contaminant identified in groundwater aquifers.
Urbanisation (and accompanying increases in wastewater generation) further exacerbates pollution, particularly in areas with inadequate sanitation and hygiene facilities. While the proportion of the global population accessing at least a basic sanitation service has increased to 68 per cent (2015), some 2.3 billion people still lack even the most minimal of facilities. Of these, the majority live in rural areas, and include some 892 million people who still must practice open defecation. Discharge of untreated effluent endangers human health and also contributes to greenhouse gas emissions, with methane emissions from raw sewage estimated to be three times higher than from conventional wastewater treatments.
Growing quantities of wastewater and pollutants in water sources not only raises acidity levels but also increases concentrations of nutrients, sediments, solvents, hydrocarbons, hormones, antibiotics, steroids and harmful pathogenic organisms. The introduction of toxins and pathogens can cause diarrhoeal diseases, cholera and dysentery, while bioaccumulation of toxins in fish and consumed by humans can lead to developmental and neurological damage.
Lower water quality also has adverse impacts on agriculture: contaminating supplies, causing land degradation, and lowering crop production. Outcomes that bring food insecurity, and exacerbate rural poverty.
The safe management of water after it has been used not only reduces pollution at source (thus protecting ecosystems while promoting better human health), but helps to alleviate water scarcity and meet an ever-growing demand for water. A ‘circular economy’ strategy of sustainable collection, treatment, reduction and re-use of wastewater can enhance energy production, recover raw materials, retain valuable nutrients, boost agricultural production and create more sustainable cities.
Sustainable solutions for wastewater management
A large-scale uptake of effective wastewater management strategies requires major investment into infrastructure. On the whole, this means improved wastewater collection and recycling facilities, as well as measures to ensure that the re-use of wastewater – along with the recovery of valuable associated resources – is not only commercially feasible, but sustainable.
Wastewater is collected through on- and off-site collection systems. As with sanitation, the main materials used for constructing pits, septic tanks and anaerobic filters are concrete, fibreglass, plastic, gravel, rocks, and bricks. Off-site collection represents a network of sewer lines, force mains, manholes and lift stations. Sewer pipes are made of sturdy materials with high resistance to deterioration, such as cast and ductile iron, concrete with plastic linings, fibreglass, plastic, vitrified clay or asbestos cement. These pipes are placed into trenches bedded by crushed rock aggregate, or sand and pea gravel. Manholes – allow access for maintenance and cleaning – are constructed from bricks, concrete barrels and fiberglass. Lift stations are used to raise wastewater from lower elevations to higher elevations, through a discharge pipe known as a force main. These stations are made up of a wet well (concrete, fibreglass, or steel) and a submersible sewage pump which pressurises the sewage. The pump housing, which contains a motor and an impeller, is made of cast iron. The hardware in lift stations is made of aluminium or stainless steel to prevent corrosion.
After collection, wastewater is transported to treatment plants, where contaminants are removed via a multi-step process. Concrete, steel and iron are essential to the construction of the buildings, pumps, valves, piping, tanks, channels and chambers used throughout the process. The pre-treatment phase, screen out large solid materials (wood, plastics), while smaller particles (sand, rock) are separated by flowing the wastewater through grit chambers. This is followed by primary treatment, whereby organic solids settle in sedimentation tanks to form sludge, which flows into aeration tanks, where microorganisms break down organic compounds.
Alternatively, water might be sprayed into the air and allowed to trickle through beds of bio-filters such as stone, gravel, coke or plastic chips, providing oxidation and bacteria to break down the organic matter. Another option is the use of lagoons (with liners made from clay, asphalt, or compacted earth), settling basins (earthen or concrete structures), and constructed wetlands (using sand and gravel as a filter bed), which rely on naturally occurring processes for treating specific industrial wastewaters. After secondary treatment, the clear liquid is sufficiently purified to be discharged back into water bodies, or re-used in industry and agriculture.
Water recycling and re-use
Depending on the type and quality of the wastewater, this may either be re-used directly (for irrigation in agriculture), treated before re-use (recycled for recharging ground water aquifers, or recirculated within industry with or without prior treatment – also referred to as industrial symbiosis. Advances in wastewater treatment allow stormwater, greywater, and wastewater to be treated to qualities acceptable for re-use in irrigation, cement production, energy generation, server cooling, or toilet flushing. The choice of on-site technologies to be used for treatment varies depending on water source and intended use, from simpler systems such as stabilisation ponds, constructed wetlands, and anaerobic digestions, to more high-tech options such as activated sludge and ozonation. The main building materials used for such systems are sand, gravel, clay, concrete, steel and asphalt. Recycled water from more high-tech treatment plants is transported to its destination through a dual piping network (to separate drinkable from non-drinkable water), constructed from steel, stainless steel, galvanised steel, iron, copper and brass.
Recovery of useful by-products
Besides water re-use, nutrients and energy can also be recovered from wastewater. Nutrients are recovered from the sludge collected during primary and secondary wastewater treatment. During anaerobic digestion, microorganisms break down the biodegradable material in closed heated digester tanks. Water is then removed from digested sludge using either a centrifuge or solar evaporation lagoons. This is essentially an extension of a classical wastewater treatment plant, so the materials used in sludge treatment are the same, ie concrete, steel, stainless and glass coated steel, due to their affordability and durability. The dried sludge cake can then be incinerated to produce heat and to power steam turbines for electricity generation (within the wastewater plant itself, or by power stations and cement producers). Treated sewage sludge also contains nutrients and, when enriched with lime, is turned into bio-solids used as soil conditioners and fertilisers in agriculture. Another by-product is biogas (methane) which can be used for process heating and electricity generation to power the wastewater treatment plant (or other industrial facilities located nearby).