Biological wastewater treatment plants are engineered environments which stimulate the growth of the purifying microorganisms that are naturally present in wastewater (section 2) without letting the grown microorganisms exit to the receiving water body. Several physical or chemical processes can be used to enhance, supplement or sometimes substitute this primary function.
3.1. Conventional activated sludge plants
The core of a wastewater treatment plant with biological removal of COD, N and P is a series of reactors which contain activated sludge, a mixture of purifying bacteria and other solids originating from the wastewater, followed by a settling tank to avoid discharge of this sludge to the receiving water body. The most basic configuration for biological removal of these three types of pollution consists of three reactors or compartments (Henze et al., 2008) (Figure 1.1). The first one receives the influent wastewater and combines it with return sludge from the settler. This unaerated compartment hardly contains any oxygen or nitrate/nitrite and is therefore called anaerobic. Such conditions allow intracellular storage of COD by PAO (Eq.
1.6). Next, the mixture of wastewater and sludge (called mixed liquor) is successively sent to an unaerated and aerated compartment, which are coupled with a recycle. In the aerated compartment, air bubbles are constantly injected to get oxygen into the water. Here, aerobic removal of the remaining COD by heterotrophs takes place, both by PAO (Eq. 1.7) and by Ordinary Heterotrophic Organisms (OHO) (Eq. 1.1). While PAO oxidise their intracellular COD storage, phosphate is stored in turn, removing it from the wastewater. Also nitrification by AOO
and NOO takes place under these aerobic conditions (Eq. 1.2 and Eq. 1.3). Part of the nitrate/nitrite that is produced in this manner, is sent to the preceding anoxic reactor via a mixed liquor recycle. Here, the absence of oxygen and still higher abundance of (intracellular) COD allows N removal via denitrification by both PAO and OHO (Eq. 1.4 and Eq. 1.7) (Barnard, 1997). Finally, the mixed liquor is directed to a settler, which is a tank where enough time is provided for the sludge to settle and as such obtain a clear top layer of water, which is suitable for discharge. Since the microorganisms grow as long as wastewater enters, excess sludge has to be regularly wasted from the system, e.g. from the bottom of the settler. The PAO in this stream are the main route for phosphorus removal from the wastewater, since they are wasted soon after the anoxic and aerobic reactors, where phosphate was stored intracellularly (Henze et al., 2008).
The part of a wastewater treatment plant which contains activated sludge can be preceded, augmented and followed by a variety of other physical, chemical and biological processes (Figure 1.1). Pre-treatment primarily aims at the removal of large solid objects and substances like plastic bags and sand which can cause clogging and damage to equipment (Tchobanoglous et al., 2014). Primary treatment via a settler is often installed right before the activated sludge line (called ‘secondary treatment’), to already remove part of the solid pollutants. Tertiary and quaternary treatment can be used to more thoroughly remove pathogens, nutrients or particular toxic substances such as pharmaceuticals. Finally, also the waste sludge can be treated further, e.g. via thickening followed by anaerobic digestion, during which the biomass is partly converted to methane (section 2.1), which can be used as a renewable fuel (Henze et al., 2008).
3.2. Alternative feeding and aeration strategies
While it is conventional to use nearly continuous flows for the influent entering the plant, for the streams between the different tanks and for the aeration air in the aerobic tank (Figure 1.1), the different biological conversions can also be performed in a single reactor instead (Figure 1.2). Sequencing batch reactors operate in cycles, starting with a feeding phase, during which influent enters. For a plant with biological P removal, no aeration should be provided during and possibly also for some time after feeding, to allow PAO to store COD intracellularly (Kuba et al., 1993). Then, the reaction phase is started, which often consists of periods with high and low (or no) aeration to allow nitrification and denitrification respectively.
Then, both feeding and aeration are turned off to allow sludge settling inside the reactor and finally, the top layer of clear water is discharged. As such, a single reactor is used for all biological conversions and settling (Artan and Orhon, 2005).
Figure 1.1. (A) A basic activated sludge configuration for biological removal of COD, N and P (Barnard, 1997) and (B) a picture of a complete wastewater treatment plant applying a similar configuration in Gdańsk to treat 92 200 m3 of sewage per day, adapted from SNG (2019).
Figure 1.2. A possible cycle configuration of a sequentially operated reactor with biological removal of COD, N and P.
One advantage of sequentially operated reactors is the flexibility they maintain after construction. For example, the aeration time can be increased to handle changing effluent limits or influent characteristics, while the tank sizes and diffusers in a continuous system fix the maximal aerated volume during construction (Irvine and Ketchum, 1988). Moreover, sequential operation improves the settling characteristics of sludge (Caluwe et al., 2017).
Another advantage is that batch feeding leads to high substrate concentrations, which enables substrate-limited reactions to occur near their maximal rate for part of the cycle time (Irvine and Ketchum, 1988), e.g. high ammonium concentrations after the feeding phase can stimulate nitrification. A related disadvantage is that the limited dilution of the influent can cause higher concentrations of inhibiting substances (Andreottola et al., 1997). Moreover, larger pumps are required because the total amount of wastewater needs to be injected in a shorter amount of time. A final disadvantage is the requirement of several reactors in parallel and/or a buffer tank, to allow treatment of a continuous flow of wastewater (Kent et al., 2018).
3.3. Alternative biomass retention methods
Alternatives to the conventional activated sludge system (3.1) do not only comprise different feeding and aeration regimes (3.2), but also different methods of biomass retention in the system. Instead of settling and recycling the thickened sludge, a membrane can be used to extract the effluent while withholding the sludge. This led to the development of so-called membrane bioreactors, which are more compact due to the better retention of biomass and thus faster reactions (Judd, 2008, Yamamoto et al., 1989). Another alternative is to add inert materials with a high surface area to a reactor to which bacteria can attach as a biofilm.
As such, microbial biomass can be efficiently retained in the system and thus more compact reactors can be obtained. Examples of modern technologies applying biofilms are moving bed biofilm reactors (Odegaard et al., 1994), integrated fixed-film activated sludge systems (Randall and Sen, 1996) and membrane aerated biofilm reactors (Martin and Nerenberg, 2012).
3.4. Alternative biological conversions
Alternatives to the conventional activated sludge system also comprise exposure to a different sequence of conditions (concentrations of COD, N compounds, P and oxygen) by using different configurations of reactors or reactor cycles. As such, different dominant microorganisms (section 2) can be selected. For example, purely anaerobic treatment is often applied to remove COD from municipal wastewater in warm climates and from concentrated industrial streams, but is still challenging for municipal wastewater in colder climates (Stazi and Tomei, 2018). N can be removed via alternative conversions as well, but this is so far only practically applicable on warm, N rich wastewaters, such as the water remaining after
anaerobic digestion of sludge or industrial wastewaters (Cao et al., 2017, Henze et al., 2008).
One alternative technology for N removal comprises nitritation-denitritation, where influent ammonium is only converted to nitrite (nitritation; Eq. 1.2) and subsequently denitrified (Eq.
1.4 and Eq. 1.7) in the same reactor by regularly turning the aeration off and adding external COD (Hellinga et al., 1998). Another alternative is a partial nitritation-anammox reactor, where influent ammonium is only partly converted to nitrite (partial nitritation) and subsequently removed together with the remaining ammonium via the anammox reaction (Eq. 1.5). Different reactor configurations are possible to obtain these alternative conversions (Lackner et al., 2014).
3.5. Aerobic granular sludge technology Principles of granule formation
The morphology of microbial aggregates like sludge flocs or biofilms depends on the operating conditions in a reactor. If the diffusion of substrates towards the aggregate is rate-limiting, the structure becomes heterogeneous and porous. This is the case if a substrate is consumed faster than it can diffuse from the water to the organism (van Loosdrecht et al., 2002). Diffusion limitation can thus be avoided by speeding up the diffusion process, e.g. by ensuring higher substrate concentrations in the water, or by slowing down the consumption, e.g. by selecting slower growing organisms. One method to do both is using a sequentially operated reactor with an unaerated feeding phase (Figure 1.2). This feeding method favours PAO and Glycogen Accumulating Organisms (GAO) instead of OHO because only they can store the influent COD under anaerobic conditions. Since a large part of the COD is present intracellularly during the aerobic and anoxic phase, no more diffusion is required before its oxidation. Moreover, PAO and GAO grow slower than OHO, with typical maximum specific growth rates of 1 d-1 (Yagci et al., 2004) versus 6 d-1 (Henze et al., 2000). The discontinuous feeding also increases the substrate concentrations in general, because the influent is diluted less with the already partly cleaned water in the reactors. Compact and large sludge aggregates, called aerobic granules (Figure 1.3), can be favoured even more if the reactor is also operated with short settling phases during which the slower settling, i.e. heterogeneous and porous, aggregates are selectively wasted from the reactor (de Kreuk and van Loosdrecht, 2004).
Figure 1.3. Microscopic image of (A) flocculent sludge and (B) granular sludge, adapted from de Kreuk et al. (2005b).
Full-scale application
Aerobic granular sludge has been applied on full-scale in sequentially operated reactors with the selective pressures described in the previous paragraph (RHDHV, 2020).
The commercial name is Nereda® and the reactors are distinguished by their simultaneous feeding and discharge phase. By slowly feeding from the bottom of the reactor via a distribution system, a semi plug flow is obtained, which pushes the upper layer of water out via several effluent weirs with minimal short-circuiting. For this reason, the reactors are taller than most activated sludge reactors, typically between 6 and 9 m (Berkhof et al., 2010, de Bruin et al., 2013). Compared to the typical loose activated sludge flocs, the obtained granules are more dense and structurally stable, leading to faster settling, which allows higher biomass concentrations and thus more compact reactors (Winkler et al., 2018). At the same time, the aerobic and anoxic environment needed for nitrification and denitrification cannot only exist when a sequence of aerated and non-aerated periods is used, but can also exist simultaneously inside the granules when oxygen is consumed before it reaches the core of the granules (de Kreuk et al., 2005a). The removal of COD, N and P in a single tank avoids the energy intensive recycle pumps required in continuously fed plants (Figure 1.1). The aerobic granular sludge technology therefore combines alternative feeding and aeration regimes (section 3.2) with the typical compactness and anoxic niches of biofilm reactors (section 3.3).
Since the first municipal installation in 2008, more and more full-scale Nereda®
reactors have been installed world-wide for both municipal and industrial wastewater treatment. Currently, more than 70 plants with this technology are built or under construction (RHDHV, 2020). The rather slow initial market uptake could be explained by the already existing wastewater treatment plants in many countries, for which replacement was not yet needed. For example, in Flanders, most pants have been built between 1992 and 2008 (VMM, 2018). To make use of the existing installations while increasing the treatment capacity,
retrofiting to sequentially operated reactors is an option (Pronk et al., 2017), but also the development of granular sludge in continuous flow systems is being investigated (Kent et al., 2018).