No one should be without clean water and sanitation
|Coastal waters are deteriorating due to pollution and eutrophication. Without concerted efforts, coastal eutrophication is expected to increase in 20 percent of large marine ecosystems by 2050.|
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.
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 reuse 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.
Besides water reuse, 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, i.e. 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).