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CHAPTER 1
Wastewater Treatment Development
Mogens Henze, Mark C.M. van Loosdrecht, George A. Ekama And Damir Brdjanovic
1.1. GLOBAL DRIVERS FOR SANITATION
In 2007, the development of sanitation was voted to be the greatest medical advance in the last 166 years in a contest run by the British Medical Journal (Ferriman, 2007). This confirms the utterly important role of proper sanitation in achieving and maintaining good public health. In many industrialized countries, wastewater is transported safely away from the households. Proper sewage treatment is however not always in place, in particular in many developing countries where sanitation coverage is, by far, less in comparison with water supply. The need for proper sanitation was made explicit in the United Nations Millennium Development Goals. Goal number 7 urges for the reduction by half of the population living without proper sanitation. Despite significant efforts, progress on sanitation targets is very slow and still lacking behind. Acknowledging the impact of sanitation on public health, poverty reduction, economic and social development and the environment, the General Assembly of the United Nations declared 2008 to be the International Year of Sanitation. The goal was to focus the world's attention on the need to start implementing proper sanitation solutions for all.
Important in this is to not only connect people to sanitation solutions, but to make this connection last in an environmentally sustainable way. Sewer systems and wastewater treatment plants have proven to be very efficient in conveying and removing pathogens, organic pollutants and nutrients. However, they require proper operation and maintenance, and a good understanding of the processes involved.
1.2 HISTORY OF WASTEWATER TREATMENT
Wastewater treatment development was the most visible in the 20th century. Sewage has for a long time been considered a potential health risk and nuisance in urban agglomerations. The fertiliser value of human excreta was already recognized in early days. The Ancient Greeks (300 BC to 500 AD) used public latrines which drained into sewers conveying the sewage and stormwater to a collection basin outside the city. From there, brick-lined conduits took the wastewater to agricultural fields which used the wastewater for irrigation and to fertilise crops and orchards. The sewers were periodically flushed with wastewater.
The Romans took this system further: in about 800 BC, they constructed the Cloaca Maxima. Initially, this central sewer system was used to drain the marsh upon which Rome was later built. By 100 AD, the system was almost complete and connections had been made to some houses. Water was supplied by an aqueduct system which carried sewage from the public baths and latrines to the sewers beneath the city and finally into the Tiber. The streets were regularly washed with water from the aqueduct system and the waste washed into the sewers.
This system worked very well because it could count on an effective government and the protection of a powerful army to maintain the far-reaching aqueducts. When the Roman Empire collapsed, their sanitary approach collapsed with it as well. The period between 450 and 1750 AD is therefore known as the "Sanitary Dark Ages" (Wolfe, 1999). During this period the main form of waste disposal was simply to dispose of it in the streets, often by emptying buckets from second-storey windows. Around 1800, a collection system appeared in many cities, driven by the city dwellers who did not want to put up with the smell anymore. It was also welcomed by the farmers around the city who found good use for this "humanure". In Amsterdam, a cart drove through the streets in which the buckets could be emptied. The cart was ironically named after a brand of eau de cologne known at that time: the Boldoot cart. However, spilling during transportation and emptying of the buckets was unavoidable, and the olfactory burden on the citizens did not decrease much. By then, plans arose for a general sewer system. High investment costs and uncertainty over flushing and maintenance of the sewers put the fast implementation on hold.
Around 1900, Mr. Liernur came up with a solution. He developed a plan for separate collection of toilet water and of grey and storm water. Toilet water was to be collected through a vacuum sewer called the Liernur system (J.M. van Bemmelen, 1868). This system found use in several European towns (Figure 1.1).
The collected sewage did not undergo any treatment. Instead, it was spread out over land as a fertilizer. However, water-logging became a major problem, and the continuous expansion of the cities made it more difficult to find sufficient land nearby. The idea that there might be better ways, using 'organisms', gradually began to emerge (Cooper, 2001).
In the United States and the United Kingdom, organisms already found their way as applied water cleaners in the so-called biological filters: biofilms on rocks in the river bed. One of the earliest biological filters, Salford near Manchester in the UK, stems from 1893. In the US the first filter was installed in 1901, in Madison, Wisconsin. Between 1895 and 1920 many were installed to treat sewage from towns and cities in the UK. This rapid application had a negative effect upon the later implementation of the activated sludge process in the UK after it was invented in 1913: investment money was already spent on the biological filters.
The activated sludge process was discovered in the UK: experiments on treating sewage in a draw-and-fill reactor (the precursor to today's sequencing batch reactor) produced a highly treated effluent. Believing that the sludge had been activated, in a manner similar to activated carbon, the process was named "activated sludge" (Ardern and Lockett, 1914).
During the first half of the 20th century, the river to which the wastewater was discharged was considered an integral part of the treatment process. The reason why 5 days is used in the biochemical oxygen demand (BOD) test is because 5 days was the longest time water spent in the rivers of the UK before it reached the sea. The book "Stream Sanitation" by Phelps (1944) uses mathematical modeling to calculate the maximum organic load from the oxygen sag curve to prevent the dissolved oxygen (DO) concentration falling below a minimum value at a point downstream of the wastewater discharge point. With the rapid growth of cities, it was soon realized that rivers could not cope with the ever increasing organic loads. As a response, the requirements increased for wastewater treatment to achieve better removal efficiencies. To reduce the oxygen demand in the river and to eliminate the toxic effect of ammonia on aquatic species, the requirement for nitrification was introduced. This led to the construction of many low-loaded trickling filter plants for organic removal and nitrification in the USA, Europe and South Africa. Anaerobic digestion was usually included in the trickling filter plants to treat the primary and trickling filter sludge produced. The discharge of nitrate from these plants was believed to be good because it provided a barrier against anaerobic conditions in the rivers and lakes. However, the trickling filters did not always nitrify very well - particularly in the winter - due to the requirement of high organic removal efficiencies prior to efficient nitrogen removal.
In the second half of the 20th century a new problem in surface water emerged: that of eutrophication. Eutrophication stands for the explosive growth of algae and other water plants due to the fertilizing effect of the nitrogen (N) and phosphorus (P) discharged to the rivers. In the 1960s it became clear that the nitrogen and phosphate also needed to be removed from the wastewater to limit eutrophication. This inspired intensive research programs and during the 1960s the fields of bacteriology and bioenergetics were applied to wastewater treatment. By applying Monod (1949) kinetics from the field of bacteriology, Downing et al. (1964) showed that nitrification depended on the maximum specific growth rate of the autotrophic nitrifying organisms which is slow compared with that of the heterotrophic organisms. For the full scale plant, this meant that the sludge age has to be long enough to achieve consistently low effluent ammonia concentrations. So successful was the use of Monod kinetics in wastewater treatment that it is still used today in all simulation models for wastewater treatment, not only to model nitrification but also many other biological processes. From bioenergetics, which was developed to a very advanced level by McCarty (1964), it was realized that the nitrate produced by nitrification could be used by some heterotrophic bacteria instead of oxygen and converted into nitrogen gas. This insight led to the nitrification-denitrification activated sludge system, in which parts of the reactor were not aerated to induce denitrification. With all this new knowledge put successfully into practice, the suspended medium activated sludge system became the preferred wastewater treatment system. The post-denitrification system, in which the non-aerated (anoxic) reactor follows the aerobic reactor, was developed by Wurhmann (1964) in Switzerland. To increase the denitrification rate in the anoxic reactor, methanol was dosed to supply the organics for the denitrification process. Because of the low nitrogen effluent values achieved with this method, this practice was widely adopted in the USA. However, methanol addition costs money, and it is rather contradictory to add organics to wastewater after first removing them. The pre-denitrification system developed by Ludzack and Ettinger (1962) formed a logical next step. In South Africa, 1972, Barnard combined the post- and pre-denitrification reactors and introduced recycle flows to control the nitrate entering the pre-denitrification reactor in the 4-stage Bardenpho system. With this development, nitrogen removal activated sludge systems became increasingly common.
A different line of development was initiated by the work of Pasveer (1959) who progressed based on the work of Ardern and Lockett. They originally designed a fill-and-draw process. Pasveer was focusing on an economical system. The ditch system he developed was based on using one treatment unit only. There was no primary settler, no secondary settler, no digester, and so forth. In the fill-and-draw process with continuous feeding, simultaneous nitrification and denitrification occurred. The simplicity and low costs led to a widespread use. Out of the Pasveer ditch system the continuous operated oxidation ditch systems evolved, based on the same principle but with a separate clarifier.
To control eutrophication, solely nitrogen removal is not sufficient. Phosphorus, mainly in the form of orthophosphate from detergents and human waste, also needed to be removed because in many ecosystems phosphorus proved to be the main limiting element for eutrophication. Unlike nitrogen, phosphorus can only be removed by converting to a solid phase. Phosphorus removal by chemical precipitation followed by tertiary filtration appeared during the 1970s. In regions where water is scarce however, like the south-western states of the USA, South Africa and Australia, indirect reuse of surface water was already high and chemical phosphate removal would cause a rapid increase in surface water salinity. Apart from the fact that salinity reduces agricultural use of surface water, its greater impact is on the durability of the water distribution system. To mitigate these impacts, water policy in South Africa in the late 1960s and early 1970s was aimed at full wastewater reclamation for redistribution to avoid both eutrophication and salination of surface water – if the high cost of chemical phosphate removal was going to be incurred, then the water may as well be reclaimed completely and returned to the distribution system rather than the environment (Bolitho, 1975; van Vuuren et al., 1975).
Biological phosphate removal is a unique biological process that has been discovered by accident. The first indication of biological phosphate removal occurring in a wastewater treatment process was described by Srinath et al., (1959) from India. They observed that sludge from a certain treatment plant exhibited excessive (more than needed for cell growth) phosphate uptake when aerated. It was shown that the phosphate uptake was a biological process (inhibition by toxic substances, oxygen requirement). Later, this so-called enhanced biological phosphate removal (EBPR) was noticed in other (plug flow) wastewater treatment plants. The first designed processes (the PhoStrip® process) for biological phosphate removal still arose from a time when the mechanism behind the process was unknown (Levin and Shapiro, 1965). In the early 1970s due to an increased demand for nitrate removal as well as for energy savings (1970s energy crisis) at several places worldwide it was discovered that biological phosphate removal could relatively easily be stimulated. For example in 1974, while optimizing nitrogen removal at the Alexandria activated sludge plant by switching aerators off at the influent end of the plant, Nicholls (1975) noted low effluent phosphorus (and nitrate) concentrations. He found very high phosphate concentrations in the sludge blanket which had settled to the floor of the reactor and into which the influent wastewater descended due to a higher density than the clear supernatant. Barnard (1976) developed the Phoredox principle for biological excess phosphate removal, which introduced anaerobic and aerobic cycling in the activated sludge system. EBPR is now an established technology, which opened the opportunity for phosphate removal and recovery without increasing salinity so that treated effluents could be returned to the environment or efficiently reused. As so often happens, new developments are found by accident and the understanding of how they work follows afterwards. It took many years of research in South Africa, Canada and Europe to fully understand and control the process and today there are still several facets about it that are not clear. However, not fully understanding the underlying principles has never stopped engineers and scientists from building and operating wastewater treatment plants.
The energy crisis in the 1970s associated with an increased demand for industrial wastewater treatment shifted attention from aerobic wastewater treatment to anaerobic wastewater treatment. The slow growth rate of methane producing bacteria had always been a limitation on the process development. For the concentrated and warmer industrial wastewaters, this was less of a problem and certainly the development of the upflow anaerobic sludge blanket reactors (UASB) by Lettinga and co-workers (Lettinga et al., 1980) meant a breakthrough for anaerobic treatment. Not only was this technology feasible for industrial wastewater treatment but also anaerobic treatment of low-strength municipal wastewater in tropical regions of South America, Africa and Asia could efficiently be introduced.
After a century of constructing wastewater treatment plants, many treatment plants that where initially built outside the urban area had become engulfed by residential areas. Expansion of plants became a problem and the engineers started to find more compact treatment options. Moreover, industry started to treat its own wastewater, and for industry, land use is even more critical than for e.g. municipalities. One successful line of development was going back to the original biofilm-based trickling reactors. A whole range of new processes was developed (biological aerated filters, fluid bed reactors, suspension reactors, biorotors, granular sludge processes or moving bed reactors) which overcame the original problems of the trickling filter process.
The development of these reactors originated from the 1970s. Another development initiated in this period only became widely introduced in the last decade: the activated sludge process with membrane separation instead of settlers.
With ever increasing effluent demands, the need arose to upgrade treatment plants instead of building new plants. Around the turn of the last century, this has led to the development of a range of new processes to be integrated in existing treatment plants. The problem tackled especially by these processes is the very high nitrogen and phosphate release during anaerobic digestion of waste activated sludge, which were traditionally recycled to the activated sludge process. Apart from struvite precipitation problems, it also results in high nutrient recycling and higher effluent nitrogen and phosphate concentrations from the activated sludge system when the dewatering liquor was recycled back to the influent. Research into this problem has led to many innovations in dewatering liquor treatment. In the Netherlands, processes were developed such as the Single reactor system for High activity Ammonium Removal Over Nitrite (SHARON®), ANaerobic AMMonia OXidation (ANAMMOX) and Biological Augmentation Batch Enhanced (BABE®) processes for improved nitrogen removal and mineral crystallization processes for phosphorus precipitation for phosphorus recovery and reuse.
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Excerpted from "Biological Wastewater Treatment"
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