By Sjoerd Kerstens
The influence wastewater treatment has had on our health and livelihoods cannot be underestimated. The process of collecting and treating wastewater has been refined significantly during the past centuries. Five factors, in particular, changed the development of wastewater treatment forever:
1. Taking waste out of town
It is believed that the Mesopotamians introduced sewer pipes made of clay around 4000 B.C, but the Romans were the first to build complete sewer systems around three thousand years later. At first, these sewers were just built for drainage during periods of heavy rainfall and to drain marshy areas. Later, the system, known as ‘Cloaca Maxima’, was built throughout Rome and transported the city’s waste to the Tiber river, without any treatment.
This system was not without risk. The citizens of Rome threw their waste into the streets and it would be partially carried away by water. It wasn’t unusual for an innocent bystander to get hit by incoming rubbish……or worse! This led to the introduction of a law, which could lead to charges for the offender.
2. Pandemics and diseases
For centuries, there was not much progress in wastewater treatment and, during the Middle Ages, the sanitation system of the Romans was forgotten. Around the Reformation, sewerage systems were introduced again in some cities. However, they were still very primitive, and the wastewater was not treated any further before it was discharged into the environment.
During the 1854 Broad Street cholera outbreak in the United Kingdom, John Snow mapped how cholera and other diseases were spread through untreated sewage that contaminated drinking water. With a growing population, the increasing pollution from industrialization, and technological advances like microscopy, the requirements for wastewater treatment became more pressing. This ultimately led to the development of aerobic biological wastewater treatment, also known as the activated sludge process, in 1913. It was first applied in the UK by Ardern and Lockett. This was truly groundbreaking in wastewater treatment: using biomass to convert organic matter and ammonium in the wastewater.
3. Pollution and enhanced nutrient removal
In the 1960’s, movements, such as those in the U.S. associated with Rachel Carson’s Silent Spring, called for more attention to environmental causes, such as pollution of lakes, rivers and coastal waters. Incidents with lakes full of dead fish and smelly rivers, both in Europe and the US, made it apparent that something had to happen. Nutrients such as nitrogen and phosphorous were found to be a major problem in uncontrolled growth of algae and other water plants (eutrophication).
As a result, regulations were set in place in the 70’s to regulate the discharge of wastewater from companies, industries and municipalities. In the US, following the 1970 creation of the U.S. Environmental Protection Agency, the Clean Water Act was introduced in 1972.
Early European water policy also began in the 1970s with the First Environmental Action Programme in 1973, followed by a first wave of legislation, starting with the 1975 Surface Water Directive, which allowed for funding for new and improved WWTPs.
From the 1990’s onwards, more and more countries started applying full Biological Nutrient Removal (BNR) systems. Nitrogen removal required the combination of nitrification (aerobic conditions) and denitrification (anoxic conditions). To remove phosphorus biologically, the biomass required exposure to anaerobic conditions (uptake of COD and release phosphate) and aerobic conditions (uptake of net phosphate). Thus, a complex, but effective system was developed comprising several compartments, internal recycle flows and (secondary) clarifiers to separate the treated effluent from the biological sludge.
However, along with this new technology, other challenges appeared, such as space availability and energy consumption.
4. The New Normal: Sustainable Wastewater Treatment
Around that same time challenges with implementation of available technologies became clear.
Lack of space
One such challenge was space availability. With increasing urbanization, available space becomes a scarce resource. The described BNR systems required quite some surface to treat the wastewater. This was mainly driven by the need to separate the effluent from the biological sludge in large (secondary) clarifiers. The flocculent structure of the sludge was the bottleneck in this separation; it limited the application of high sludge concentrations in the reactor, which increased reactor sizes, but also resulted in large clarifiers. Therefore, technologies were developed that applied membranes (MBR) or carrier materials for space efficient sludge-water separation.
Climate and energy
However, these technologies moved the challenge for space to a much more prominent challenge, namely climate & energy. The operation of MBR and systems applying carrier materials required much more energy compared to existing biological systems. Further, chemical consumption increased for membrane maintenance, to enhance denitrification and to remove phosphorus.
Therefore the need arose to develop a technology that allowed for (1) small footprint (high MLSS, with short sedimentation times), (2) simple 1-compartment tank systems, and (3) low energy and no/minimum chemical requirement by applying biological BNR.
In 1993, Professor Mark van Loosdrecht from Delft University of Technology started working on aerobic granular sludge technology. Van Loosdrecht successfully converted flocculent activated sludge into aerobic granular sludge. The advantage of granular sludge was that it settles much faster than flocculent sludge. By applying a batch type of system, all processes (including settling) could take place in one reactor. Due to the excellent settling properties, not only could much higher (double!) sludge concentrations be maintained in the reactor, but also settling times are much shorter and are almost negligible on a total cycle time.
The developed sludge comprised a layered structure of different types of organisms, but without the need for (plastic) carriers. Varying oxygen gradients in the granules resulted in a biofilm-like layered structure that promotes simultaneous nitrification and denitrification. Moreover, by alternating anaerobic (feeding) conditions with aerobic conditions, Bio-P is achieved in the granules.
Together with Royal HaskoningDHV and the support of Dutch Water utilities, Nereda Aerobic Granular Sludge technology was developed.
The result is a biological system that has a low footprint requirement, low energy, low lifecycle costs and chemicals requirement and a very simple configuration. The first plant with this technology was opened in 2005. Today there are almost 90 projects in contract, of which nearly 50 are Nereda plants in operation with more than 30 on the way.
5. Future outlook
The future of wastewater treatment holds challenges, but also many opportunities. Some of the drivers for further developments include advanced treatment, resource recovery and production.
Micropollutants from pharmaceuticals, such as antibiotics, household chemicals and personal care products, often appear in water. They have become an increasing concern over recent years but most WWTP have still not yet been upgraded for better management of the removal of micropollutants. Integration of micropollutant removal in Nereda is currently in development.
Phosphorus is a scarce yet essential resource, which is why phosphorus removal and recovery from wastewater treatment are becoming increasingly important. The high Bio-P uptake in aerobic granular sludge offers a range of possibilities for phosphorus recovery.
Water scarcity across the globe is increasingly becoming a problem. Currently around 1.2 billion people live in areas of water scarcity and the UN estimates could increase to as many as 5 billion people by 2050.
There is increasing pressure to move towards sustainable raw materials and away from oil-based products. Kaumera, a raw biopolymer, which can be extracted from aerobic granular sludge, shows promising future application possibilities as a replacement for oil-based polymers in areas such as agriculture, horticulture and construction. Furthermore, 20-35% less sludge needs to be disposed of, which also reduces the WWTP’s energy consumption.