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CHAPTER 1
Nutrient removal
Francesco Fatone, Juan A. Baeza, Damien Batstone, Grzegorz Cema, Dafne Crutchik, Ruben Diez-Montero, Tim Huelsen, Gerasimos Lyberatos, Andrew McLeod, Anuska Mosquera-Corral, Adrian Oehmen, Elzbieta Plaza, Daniele Renzi, Ana Soares and Inaki Tejero
1.1 INTRODUCTION
1.1.1 Nutrient management regulation and implications on energy consumptions
After decades from the Urban Wastewater Treatment Directive (271/91/EEC), nutrient pollution resulting from excess nitrogen (N) and phosphorus (P) is still a leading cause of degradation of water quality in Europe (European Commission – JRC, 2014). More stringent nutrient management practices and regulations are therefore needed and have been undertaken. Considering for example the recently identified "ecoregions" in the USA (WERF, 2010), it is clear that current trends are establishing very low standard for in-stream concentrations of N and P which will result in standard for nutrient discharge in sensitive watersheds much lower than 10 mgN/L and 1 mgP/L set by the Directive 271/91/EEC. Technology-based nutrient limits at or near the limit of technology (LOT) are being considered in several regions in the United States and abroad. The LOT for total nitrogen (TN) is typically defined as 3.0 mg/L and total phosphorus (TP) of 0.1 mg/L or the mass-load-based equivalent at the design capacity of the wastewater treatment plant. In some regions, especially sensitive watersheds or ecosystems, TP limits much less than 0.1 mg/L are being considered.
In Europe a recent survey carried out within the Water_2020 network (ES1202 COST Action) concerned the most sensitive areas, where special local nutrient management legislation is applied (Table 1.1). The Water_2020 partners pointed out that the lowest limits on both total nitrogen and phosphorus are set in Finland for the Helsinki Region wastewater treatment plant. Here, the standards of 4.5 mgN/L and 0.3 mgP/L must be achieved to discharge into the eutrophicated Baltic Sea. On the other hand, standard for P discharge in very sensitive watershed are already as low as 0.1 mgP/L and further lowering around Europe is planned.
When considering the questions "how low can we go" and "what is stopping us from going lower" (WERF, 2010), we must consider that the nutrient challenge consists in striking the balance between nutrient removal, greenhouse gas emissions, receiving water quality, and costs, so a triple bottom line (TPL) analysis is needed to include environmental, economic, and social pillars (Falk et al. 2013).
To achieve the new, lower effluent limits that are close to the technology-best-achievable performance, facilities have begun to look beyond traditional treatment technologies (U.S. EPA, 2007). Nutrient removal processes could be classified in three "levels" of effluent concentration: i) achievable with conventional nutrient removal technologies (8 mgN/L and 1 mgP/L); ii) enhanced removal requires tertiary treatment and chemical addition to achieve low concentrations (3 mgN/L and 0.1 mgP/L); iii) requires state-of-theart technology and enhanced/optimized treatment operation, especially to simultaneously achieve both the very low N and P levels (1 mgN/L and 0.01 mgP/L).
The more is the nutrient removal technology complexity, the more is the energy consumption and the Greenhouse gas (GHG) emissions, which largest contributors were found to be energy related (Falk et al. 2013) (Table 1.2).
Therefore, energy efficiency in nutrient removal in wastewater treatment plants (WWTPs) is clearly one of the key pillar to consider for the water-energy-carbon nexus.
1.1.2 Biological Nutrients Removal processes: microbial and energy overview
In recent times, there has been an increased emphasis on increasing the efficiency of BNR processes and reducing the operational costs. One means of improving the cost-effectiveness is by employing short-cut nitrogen removal, or nitrogen removal via the nitrite pathway (Table 1.3). This involves aerobic nitritation by AOBs coupled with anoxic denitritation by denitrifiers, thus necessitating the limitation of NOB growth and activity. Some WWTP operational conditions are known to favour AOB at the expense of NOB, such as the higher growth rate of AOB at temperatures higher than 25°C (Hellinga et al. 1998), as well as the lower affinity of NOB for oxygen, where a low dissolved oxygen (DO) concentration will favour nitrite accumulation instead of nitrate. Short-cut nitrogen removal reduces the oxygen demand of the WWTP by 25% through eliminating the need to oxidise nitrite to nitrate, while simultaneously reducing the COD needed for denitrification by 40% through eliminating the need to reduced nitrate to nitrite. Aeration is widely considered to be one of the main energetic costs associated with WWTP operation, while the external dosing of COD sources also increases costs due to the expense associated with the COD supply as well as the increased sludge production, where sludge processing and disposal also represents one of the main operational costs associated with WWTPs.
In Table 1.3, a comparison is made between the biomass production, COD and oxygen requirements associated with wastewater treatment plant processes performing COD, N and P removal, as well as their respective nitrogen and phosphorus removal levels (standardized per mg of nitrogen removed). It is clear from Table 1.3 the savings in COD and oxygen requirements as well as the reduced sludge production achievable through short-cut nitrogen removal as compared to conventional nitrification/denitrification.
The anaerobic ammonia oxidation (Anammox) process has also attracted much attention in recent years, since it achieves N removal from wastewater with even further reductions in aeration and COD requirements, as well as sludge production (Strous et al. 1997, 1999), as can be observed in Table 1.3. In Anammox, ammonium is oxidized directly to dinitrogen gas using nitrite as the electron acceptor. Anammox bacteria are autotrophic and require low oxygen concentrations to survive, or are otherwise rapidly outcompeted by comparatively faster-growing nitrifiers. Partial nitritation is often employed prior to the Anammox process in order to generate sufficient nitrite. The molar nitrite/ammonia ratio is about 1.3 in the Anammox reaction, due to the simultaneous production of a small amount of nitrate (Strous et al. 1997). Anammox is becoming increasingly employed during the treatment of high strength ammoniumcontaining wastewaters with low COD content, including the supernatant from sludge digesters, landfill leachates and industrial wastewaters (Wett 2006; van der Star et al. 2007; Ganigue et al. 2009; Lackner et al. 2014) due to its high potential to increase the cost-effectiveness in WWTPs, although it remains a sensitive process to operate in practice, since it is prone to inhibition by toxic compounds and the slow biomass growth rate results in lengthy start-up/recovery periods.
Phosphorus is another key nutrient that stimulates the growth of toxic cyanobacteria (blue-green algae), and has been found to often be the limiting nutrient leading to eutrophication (Mainstone & Parr 2002). While P can be removed via chemical precipitation, enhanced biological phosphorus removal (EBPR) processes promote the removal of phosphorus from wastewater without the need for the addition of these chemicals, leading to a more cost-effective option for phosphorus removal when operated successfully. The addition of chemicals not only increases the operational costs due to the demand of reagents, but also increases the sludge production. In the EBPR process, the group of organisms primarily responsible for phosphorus removal are known as the PAOs. In order to promote the development of PAOs and, consequently, P removal, anaerobic followed by anoxic and/or aerobic conditions are generally employed, thus combining very well with BNR processes designed for biological nitrogen removal. PAOs are able to take up carbon sources such as volatile fatty acids (VFA) anaerobically and store them as polyhydroxyalkanoates (PHA), providing them a selective advantage over most ordinary heterotrophs. However, a competitor group of organisms known as glycogen accumulating organisms (GAOs) are also capable of anaerobic VFA uptake and therefore can also be enriched under similar conditions as PAOs, consuming the generally limited VFA supply without contributing to P removal.
While the majority of the P is generally taken up under aerobic conditions in most conventional EBPR processes, simultaneous denitrification and P removal can save on aeration, minimise sludge production and reduce the demand of readily biodegradable COD, which is often-limiting (Table 1.4). The combination of nitritation with EBPR via denitritation can lead to further savings in both COD and oxygen demands (Table 1.4). Nevertheless nitrite accumulation (in the form of free nitrous acid) is known to inhibit P uptake by Polyphosphate Accumulating Organisms (PAOs) when present at high levels (Saito et al. 2004; Zhou et al. 2007), and can lead to the undesirable production of N2O (a powerful greenhouse gas) (Zhou et al. 2008). This is of particular relevance for sludge unacclimatized to high nitrite levels (Zhou et al. 2011). BNR processes applying denitritation and P removal should avoid excessive levels of nitrite accumulation to prevent N2O accumulation and maximize P removal. While the potential for increasing the cost-effectiveness of EBPR systems through increasing the P fraction removed anoxically is high, this still remains a challenge to achieve in practice since the aerobic zone can only be eliminated in segregated sludge systems and PAOs grow more quickly aerobically than anoxically, lowering their denitrification capacity.
Furthermore it should be noted that the estimates presented in Table 1.4 neglect the growth of Glycogen Accumulating Organisms (GAOs), which would increase the COD demand and therefore sludge production in systems where they proliferate. Some factors favouring the growth of GAOs include high temperature, low pH and a very high acetate/propionate ratio (Lopez-Vazquez et al. 2009), as well as high DO concentrations (Carvalheira et al. 2014). In this sense, the increasing practice of operating BNR plants at low DO levels can be beneficial not only for reducing aeration costs, but also minimizing the growth of GAOs. Furthermore, there are differences between the different groups of PAOs and GAOs related to their capacity to denitrify. Experimental studies have shown that some clades of Accumulibacter (a common PAOs in BNR plants) are able to denitrify from nitrate onwards, while essentially all Accumulibacter clades denitrify from nitrite onwards (Carvalho et al. 2007; Flowers et al. 2009; Guisasola et al. 2009; Oehmen et al. 2010a). With respect to different microbial groups of GAOs present in WWTPs, some Competibacter sub-groups have been found to be able to reduce nitrate and nitrite, where others are able to reduce nitrate only or not denitrify at all, while Defluviicoccus Cluster I was able to reduce nitrate, but not nitrite and Defluviicoccus Cluster II were unable to denitrify (Kong et al. 2006; Burow et al. 2007). Overall, it is clear that comparatively few GAOs groups are able to metabolise nitrite as compared to PAOs, suggesting that combining nitrogen removal via the nitrite pathway with denitrifying EBPR is a potential means of eliminating GAOs (Taya et al. 2013), leading to further savings in COD, aeration and sludge production as compared to P removal via nitrate or oxygen.
1.2 REDUCING ENERGY FOOTPRINT NOW, BY RETROFITTING
Water_2020 members have been working to retrofit existing WWTP by applying innovative energy efficient nutrient removal technologies.
1.2.1 Sidestream technologies/systems
1.2.1.1 ELAN system: Pilot study and full scale retrofitting
The ELAN® (Autotrophic Nitrogen Removal in Spanish) process has been developed by the company FCC Aqualia (Spain) with the know-how of the University of Santiago de Compostela by means of collaborative work that was started in the year 2009 (Vazquez-Padin et al. 2014a). It is based on combining the partial nitrification and anammox processes (1.1), for nitrogen removal from wastewater, in a single aerobic unit where the biomass is grown in the form of granules (Vazquez-Padin et al. 2014b).
NH4+ + 0.85O2 + 1.11HCO3- -> 0.44N2 + 0.11NO3- + 2.56H2O + 1.11CO2 (1.1)
This process has been successfully applied, at pilot scale, for the treatment of the reject water from WWTPs characterized by an ammonia concentration of 0.5–1.5 g NH4+-N/L and a temperature in a range of 18–31°C. Two locations have been tested in two WWTPs located in Galicia. In one case the used reject water was collected from an anaerobic digester treating the sludge from the plant (WWTP of Vigo) and in the other case sludge was co-digested together with agricultural wastes (WWTP of Guillarei).
At the moment a full scale plant of 115 m3 is being started up for the treatment of the reject water in the STP of Guillarei. This reactor has been designed to treat 67 kg N/d corresponding to 23% of the total nitrogen treated in the WWTP (Vazquez-Padin et al. 2014b). Once this reactor is implemented, an improvement of the quality of the produced effluent is expected by means of the decrease from 15 to 13 g TN/m3 in the effluent of the plant.
Special features of the process
The ELAN® process is specially appropriated for the removal of nitrogen from wastewater streams characterized by the low BOD5/TN content which would require external addition of carbon source for the performance of the conventional nitrification-denitrification processes. Taking into account that the anammox process performs in optimal conditions at temperatures in the range of the mesophilic values, the effluents produced from sludge anaerobic digesters in STPs are appropriated to be treated by the ELAN® process. Its application to treat this sidestream will allow for the obtaining of a better quality effluent, in terms of nitrogen content, of the STP as less nitrogen has to be treated in the mainstream treatment system. This process can be easily integrated in the plant as an extra unit after the anaerobic digester provided with a previous equalization tank.
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Excerpted from "Innovative Wastewater Treatment & Resource Recovery Technologies"
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