THE ASSESSMENT OF RISK IN SELECTION OF TREATMENT OF POST THP SLUDGE LIQUORS

Transcription

1 THE ASSESSMENT OF RISK IN SELECTION OF TREATMENT OF POST THP SLUDGE LIQUORS Abstract Nemtsov, D. 1, Poznanska-Chapman, K. 2, and Hopkins, S. 1 Mott MacDonald Bentley, 2 Mott MacDonald, United Kingdom The conversion of a Dŵr Cymru Welsh Water WwTW to a regional Sludge Treatment Centre (STC) using a Thermal Hydrolysis Process (THP) will result in an approximately threefold increase in sludge throughput. Thermal hydrolysis of sludge has significant impact on the quantity and composition of sludge liquors, in particular: Ammoniacal Nitrogen (Amm.N), COD, phosphorus and inhibitory substances. The importance of defining the impact of the predicted increase in Amm.N load on the main process at the WwTW was identified during the design development. A new Liquor Treatment Processes (LTP) will be implemented to treat the increased Amm.N loads. This paper explores the methodology for determining the liquor composition and the risks associated with selection of an appropriate LTP. The risks explored are: uncertainty in Amm.N and P load, impact of P on the main process and reliability and robustness of LTP. Several commercially available Liquor Treatment Processes were considered: Anammox, nitritation/denitritation and nitrification/denitrification. The identified risks are compared to potential OPEX savings. Keywords sludge liquor treatment, ammonia removal, phosphorus removal, Anammox, Thermal Hydrolysis, digested sludge centrate, nitritation Introduction Dŵr Cymru Welsh Water's (DCWW) strategy for reducing carbon footprint and increasing operational efficiency includes the conversion of an existing WwTW into a large Sludge Treatment Centre (STC) treating sewage sludge from across the North Wales region. The existing sludge treatment process comprises anaerobic digestion (AD) with lime addition; this is to be converted to Advanced AD (AAD) with THP and also Biomethane to Grid. As a consequence of the conversion, not only will there be an increased amount of sludge to be treated, but also significant change in quantity and quality of sludge liquors which will impact upon the performance of the WwTW. This paper focuses upon the risks associated with treatment of sludge liquors and how these are balanced against the strategic objective of increasing operational efficiency by reducing costs. Figure 1 shows the geographic relationship between the new STC where the AAD plant is to be developed and eight dewatering centres where sludge is screened and dewatered for export to the STC. Numerous satellite sites export liquid sludge to the dewatering centres and STC.

2 Figure 1: AAD sludge export diagram (size proportional to sludge throughput) The average design capacity of the new STC is approximately 20,000 tds/y. The proportion of untreated sludge generated at the STC, dewatering centres and satellite sites is summarised in Table 1. The vast majority of sludge to be treated in the STC is non-indigenous and represents a considerable change in process at the WwTW. Table 1: Sludge Throughput at New STC Annual Average Design Throughput (tds/y) STC indigenous sludge Dewatering Centres Satellite sites Total Peak (1) Percentage of total 1 includes allowance for growth, seasonality and uncertainty Figure 2 is a Process Block Diagram (PBD) of the wastewater treatment process at the STC prior to introduction of AAD. Liquors arising from dewatering are treated within the existing Activated Sludge Process (ASP). The works comfortably meets the Amm.N limit of 10 mg/l (95%ile) and complies with total phosphorus consent of 1mg/l (annual average).

3 Figure 2: PBD of existing WwTW and AD plant Figure 3 includes the process additions for the AAD with the import of dewatered sludge cake. The AAD process increases the quantity and strength of liquors from dewatering. The risks assessed when making the decision to introduce an LTP to treat these liquors are discussed in the paper. The liquors arising from thickening indigenous sludge remain largely unchanged in quantity and characteristics and will continue to be treated in the existing ASP. Figure 3: PBD of proposed STC including AAD Sludge Treatment Centre Risks Sludge Quantity When converting an existing WwTW into a regional STC, perhaps the first consideration is the anticipated quantity of sludge to be treated. A common approach is to use per capita dry solids (g/ca/d) estimates derived from water companies design standards, which will typically range from 70 to around 80 g/ca/d depending on wastewater treatment process. However, experience suggests that when this approach is adopted, treatment capacity can be over sized, with consequent impact on the economic benefit of the project. Dry solids loads estimated based on measurements can be 15% to 20% lower than those derived from the design standards or even as low as 60 g/ca/d. This has implications for the estimation of quantity of sludge liquors and sizing a LTP.

4 In this project, to mitigate the risk of over estimation of sludge quantities, data logging was introduced for both sludge imported to existing regional sites and biosolids exported for use in agriculture. Mass balances, comparing exports, imports and anticipated indigenous sludge production reduced the estimated required treatment capacity from tds/y to tds/y, a reduction of 17%. The analysis of logged sludge data supported experience of STCs being oversized when throughput is estimated using conventional design standards. A further advantage in logging sludge and developing mass balances over a 2 to 3 year period is that the seasonal variations can be directly measured. This is particularly relevant for estimates of sludge production from areas with significant tourist industry. The logged sludge data indicated lower sludge flows when compared to an alternative approach of using transient population estimates from the June returns. By examining the variance in the data using different methodologies, the confidence in the sludge flow data was also estimated and helped with an assessment of the liquor characteristics. Another factor in design of AAD and estimation of composition of sludge liquors is the composition of the sludge; expressed as proportions of primary solids (i.e. separated from wastewater during primary settlement), biological solids and chemical sludge. In the absence of actual measurements at individual satellite and dewatering sites, a proportion of different types of solids in sludge was approximated by theoretical calculations of primary and biological solids for individual sites using standard sludge yields. The potential for the actual primary/biological solids proportion to deviate from those calculated using design standards is a risk to digestate and biogas production estimates and, in particular, the ammonia and phosphorus load in liquors arising from dewatering the digestate. This risk is compounded by uncertainty regarding the biological degradation of sludge that will be stored at satellite and dewatering sites for considerable periods prior to export to the STC. Storage of up to several weeks was anticipated at some smaller sites Having identified the risk associated with determining sludge characteristics from a theoretical approach, undertaking lab-scale trials so that actual composition of sludge liquors could be determined was considered. The main difficulty with this approach would be to generate a representative sludge sample that would be continuously fed to the lab-scale AAD throughout the trials. This, would necessitate gathering sludge samples from satellite sites over an extended area, blending them in proportion to their plant throughput, and transporting them to a suitably equipped laboratory. This would be difficult and costly. However, even if this could be achieved, the current processes at each dewatering site will change once the new STC is completed, in particular liming will cease. Also, to generate a representative sample of sludge liquors a very long AAD trial period would be needed. This risk mitigation strategy was reviewed in the context of cost vs risk reduction. On balance, it was decided the costs exceeded the risk reduction and significant risks associated with the ammonia and phosphorus loads in sludge liquors would be managed in selection of the LTP. Although a comprehensive sampling and digestion regime was not undertaken, two specific risks associated with phosphorus were examined: the phosphorus release test on the sludge taken from a WwTW that includes EBPR (Enhanced Biological Phosphorus Removal) and will in the future export its sludge to the new STC, and an apparent uncertainty about the amount of ferric sulphate used at the WwTW at the future STC implying EBPR activity in the ASP stream. The conclusion drawn from these tests was that there was no evidence of EBPR activity in the ASP plant.

5 Quantity and Quality of Sludge Liquors The uncertainties in the estimate of quantity and quality of sludge to be treated at the new STC inevitably affects the accuracy of estimation of flow and composition of sludge liquors, especially those from dewatering of the digested sludge. Liquor flows were estimated using a simple flow balance around the dewatering plant but determination of their composition has been more challenging as it involved prediction of the impact of the thermally hydrolysed sludge. The method applied involved theoretical estimation of the main determinants for the current situation. This was then validated by measurements on site and then extrapolated into the future using data from other THP plants operated by DCWW. High VSS destruction in the digesters, resulting from the thermal hydrolysis, accompanied by a high DS concentration in the digester feed, allow the throughput of the existing digesters to be significantly increased. Consequently, ammonia in the digesters and in sludge liquors from the dewatering will increase. The DS concentration in the digester feed is limited to 10.5% so that the risk of digestion inhibition by ammonium is minimised. Whilst there are many publications quoting concentration of Amm.N in the post-thp sludge liquors, they do not provide any information on polyelectrolyte dose and water consumed during dewatering, which makes it difficult to use them for specific applications. In this project, the Amm.N concentration in the post digestion sludge liquors has been determined based on earlier estimation of composition of raw sludge, in particular its biological sludge content. It has been assumed that nitrogen accounts for approximately 3.5% of VSS in primary solids and 6.5% of biological VSS. A proportion of nitrogen would be released from the solid to the liquid phase as a result of VSS destruction during digestion. Our previous THP experience, operational data from DCWW and some additional measurements on site have been used to adjust the theoretically derived content of ammonia in liquors. The average Amm.N concentration in the undiluted centrate, i.e. without dilution effect of water used for polyelectrolyte preparation and dosing, was estimated to be mg/l. Alkalinity is an important parameter that needs to be taken into account when considering removal of ammonia and phosphorus. Alkalinity in the post digestion liquors, though high and proportional to Amm.N concentration (molar ratio of 1:1 as HCO3 - : Amm.N was assumed), is not sufficient to oxidise ammonia by a conventional nitrification route. Estimation of phosphorus content in the post THP sludge liquors was more difficult than that of ammonia, not only because of rather limited data in the literature but also the complex nature of the phosphorus fate during biological wastewater treatment and digestion of sludge. Solids in primary sludge typically contain low quantities of phosphorus (less than 1% of VSS) and are believed to contain various metals such as calcium and aluminium which play an important role in phosphorus fixation in the solids phase during anaerobic digestion (Bungay et al., 2007). Moreover, as at this WwTW, ferric sulphate is being added upstream of the primary settlement tanks, the phosphorus precipitated from wastewater is likely to remain in the solid phase during sludge digestion. Also, any excess iron (that has not reacted with phosphorus in wastewater) would bind some soluble phosphorus arising from destruction of biological sludge VSS. However, the ratio of fixed to released phosphorus could not be reliably determined. Any form of extrapolation from the existing post digestion liquors to the future ones was considered to be unsuitable because the current liming of sludge is likely to affect this ratio. It is known that at works with a very high proportion of surplus activated sludge, especially with EBPR sludge, phosphorus concentrations in the sludge liquors after digestion are very high, however there are also situations where if a large proportion of phosphorus remains chemically bound in sludge not enough of it is available for biomass growth in the LTP, which necessitates addition of phosphorus to side stream treatment of liquors (Butt, 2016). As no evidence of EBPR mechanism was found at the

6 existing ASP and only about 2% of the total DS sludge load to be treated at the STC comes from a site with EBPR, no excessive release of phosphorus into the liquid phase is expected during sludge digestion. Hence, the levels of phosphorus in the post digestion liquors at this WwTW are expected to be between these two extreme scenarios, i.e. 80 mg/l in the undiluted centrate which compares well with values measured at another DCWW site with THP. The COD load in sludge liquors discharged to WwTW will increase once the THP plant is operational, due to relatively high VFA (volatile fatty acids) levels and a significant proportion of a non-biodegradable COD, an unavoidable consequence of the thermal hydrolysis. It has been estimated that the WwTW final effluent COD will increase but will not exceed the discharge consent. It should be emphasized that as much as 40% of the total COD could be inert and inhibitory to biological treatment (Figdore, 2011) as discussed below. Commissioning/ Transition Period The configuration of the existing WwTW heavily influences the commissioning strategy for the introduction of AAD. The strategy adopted at the STC examined in this paper was for the existing two digesters to be sequentially converted to digest hydrolysed sludge. This strategy results in a considerable variation in the quantity and quality of liquors to be treated as AAD will be introduced over an extended period. Close consideration was given to the implications and risks of the commissioning strategy on the choice of LTP technology. A further consideration is the availability during commissioning of heat for those LTP technologies that require elevated temperature in the bioreactors. Liquor Treatment Options Use of Existing WwTW Treatment Capacity If post-digestion liquors were returned untreated to the ASP, the ammonia load on this stream would increase by 118% compared to the current loads. The aeration capacity is insufficient to satisfy the additional oxygen demand. Also, there would be a substantial shortfall in alkalinity. Notwithstanding the concerns expressed above, there is sufficient ASP volume available to treat the increased ammonia loads. This potential capacity is, however, limited by an inefficient flow split to the ASP lanes, limited size of the anoxic zones. The option analysis therefore took into account the costs associated with increasing the aeration capacity of the existing process, alkalinity addition, improving deficiencies identified in the existing ASP stream as well as deviation of the BOD:Amm.N ratio from typical values. To minimize the Capex, Opex and associated risks/uncertainties it was concluded that a dedicated LTP treating all liquors from post digestion dewatering was required. The Amm.N load from the LTP returned to the existing process will be reduced by 80% and will be no more than currently returned from dewatering the AD digestate. Also, there would be sufficient alkalinity in the wastewater to satisfy the demands of chemical dosing for phosphorus precipitation. On completion of the AAD project, it is predicted that the return liquors will have an approximately fourfold increase in total phosphorus levels.

7 Although the inclusion of the ammonia removing LTP makes the need to address the deficiencies in the existing ASP and associated equipment as part of the AAD project less urgent, there is still a need to provide good conditions for chemical phosphorus removal. Phosphorus Risk Mitigation Strategy A risk mitigation strategy has been developed to address removal of the future phosphorous loads in case removal in the WwTW proves difficult. The existing ferric sulphate dosing system on the main WwTW can be upgraded, however, this option bears some risk as it will increase the amount of sludge within the plant with the potential to breach the iron consent. Also, if phosphorous concentrations are found to be higher than anticipated, the subsequent increased dose of chemicals may cause a shortfall in alkalinity in the ASP stream. Another option would be to remove the phosphorous from the post AAD sludge liquors at the point of discharge to the main works. This would require an additional mixing and flocculation facility. Alternatively, ferric salts could be dosed directly into the digesters. Iron dosing into the digester increases the P content of biosolids with consequential effects on agricultural application rates. There is a risk that excessive iron dosing can result in P-deficiency for the microbial biomass in the digester and reduce biogas production. Dosing magnesium salts to the aerated post-digestion tank would stimulate struvite formation in this tank for removal. The MgO as Mg source, would increase the alkalinity of liquors which would assist the LTP plant where Amm.N is removed. The metal salts (iron or aluminium) could be dosed directly into the centrifuge feed pipes. This arrangement would allow a dose-response relationship to be established relatively quickly ensuring that phosphorous is kept within the sludge. On the negative side, dosing significant amounts of ferric salt could introduce several issues downstream of the process such as alkalinity reduction. Proprietary P recovery systems using struvite (MgNH4PO4 6H2O) precipitation have not been selected for this application as the estimation of phosphorus load in the digested sludge liquors is uncertain and, moreover, is expected to be below the level which is considered to be economical from the revenue point of view. LTP Technologies As the amount of phosphorus in the sludge liquors is expected to be too low to justify the costs of phosphorus recovery in the form of struvite and there is uncertainty (i.e. risk) about its load, the main emphasis was on providing removal of ammonia. The risk strategies discussed above may be adopted in the event that the phosphorus in the sludge liquors is too high to be removed by chemical precipitation at the WwTW. Following outline review of various technologies, it was agreed with DCWW that only options with biological ammonia removal would be considered, physical and chemical technologies being considered insufficiently developed for further consideration. A wide range of processes have been considered. On the one hand, well established processes using nitrification and denitrification but with high Opex and large bioreactors and footprint, whilst on the other, more sophisticated technologies using relatively novel biochemical paths (nitritation-denitritation and Anammox based technologies) characterised by lower Opex and footprint but higher process risks. All technologies considered have been assumed to treat only post-digestion sludge liquors.

8 Conventional nitrification/denitrification in unheated or heated bioreactors is well understood, being commonly used within wastewater treatment in various bioreactor configurations (Jardin et al., 2006). The use of heated bioreactors, further referred to as a high-rate system, increases the rate of biochemical reactions but requires external source of heat. These processes, if applied to post AAD liquors need addition of organic carbon and alkalinity and consume a large amount of energy for aeration. The bioreactors are relatively large and so is the footprint of a plant. Nitritation/denitritation is carried out in a chemostat bioreactor at 35ºC, whereby the demand for oxygen and carbon source, though relatively high, is reduced in comparison with conventional technologies (Hellinga et al., 1998). A heat exchanger is required to maintain the temperature at a desired level. However, the bioreactors are smaller than those needed by conventional technologies. Partial nitritation followed by an Anammox process, all carried out in one bioreactor, is designed to remove primarily ammonia (and TN) rather than BOD and COD (as in the more conventional technologies). No organic carbon or additional alkalinity is needed, if ammonia removal is less than 80%, albeit this is accompanied by the introduction of process risks (discussed below). The footprint of the Anammox based plant, depending on selected technology, can be very small. Features and Risks of LTP Options Ability to Reduce Ammonia Loads All technologies considered can remove ammonia from the post digestion liquors to a high degree; in excess of 97% for conventional and high-rate activated sludge plants, providing sufficient organic carbon, alkalinity and aeration capacity is available. The Anammox process achieves lower reductions, up to about 80% without any addition of chemicals whilst some more removal is achievable if alkalinity is added. Applications of Processes Considered Although activated sludge systems, particularly in the form of SBR (Sequencing Batch Reactors), have been widely used to treat sludge liquors, their use is limited to treatment of all liquors from wastewater treatment works rather than just ammonia rich post digestion liquors. Heated, high-rate activated sludge systems (such as Amtreat) are better suited for post digestion liquors and there is one site in the UK where post-thp liquors are treated by this system though in a mixture with less concentrated liquors arising of sludge thickening prior to THP. Nitritation/denitritation systems (such as Sharon) have been designed to treat liquors with high ammonia concentrations and have also been used to treat liquors from THP AAD. With a very low requirement for aeration energy and no need for chemicals, Anammox based systems are particularly well suited to treat ammonia rich liquors. On the other hand, the process is sensitive to high suspended solids and organic matter content (BOD, COD) which may cause process imbalance as heterotrophs can form an excessive proportion of the biomass. Also, the Anammox based technologies are prone to inhibition by THP by-products, though there are means of alleviating this risk. Inhibition by THP By-Products Recent findings indicate that sludge liquors generated during dewatering of post THP digested sludge contain compounds that are inhibitory to the biomass used in liquor treatment, most likely to AOB (Ammonia Oxidising Bacteria). The inhibitory compounds have not been identified but are thought to be associated with soluble inert COD (Figdore, 2011) which is a result of the Maillard reaction during thermal hydrolysis. Inhibition has been observed in Anammox based plants; there is no indication that such problems occur in more conventional technologies, even though AOB play an important role in all of them. Only the suppliers of the Anammox based technologies specified a need to alleviate the risk

9 of inhibition and all of them proposed dilution of sludge liquors with works final effluent. The impact of dilution is primarily on bioreactor size, availability of dilutant and decrease of ammonia concentration affecting reaction rates. If warm final effluent is available, which will be the case at this STC (from THP sludge coolers), its use for dilution would help to maintain the temperature in the Anammox bioreactor at the desired level. On balance, the risk of post THP inhibition, if managed through dilution, would not favour one technology over another except when a warm dilutant is available (the Anammox based plant would benefit from this). Variations in Flow Rate and Loads All biological processes benefit from continuous and stable flows and loads to ensure constant environmental conditions for the growth of the microorganisms. Variations in flows and loads of liquors presented to an LTP for treatment can occur for a number of reasons and over short or longer periods. At the STC examined in this paper, short term (1-2 days) variations caused by scheduling of imported sludge cake, dewatering approaches and routine maintenance are accommodated within buffering capacity and, in particular a liquor buffer tank of one day retention capacity immediately upstream of the LTP. Longer term variations cannot be practically accommodated by buffering. Each of the technologies under consideration react differently to variation either side of the average flow and load. This is a consequence of the rate of growth of the biomass utilised. Hence, means to control the main process parameters are essential to enable the liquor treatment systems to perform well under a wide range of flows and loads. Process Stability/Robustness/Control Each of the processes under consideration have different approaches to process control as a consequence of the different biological pathways adopted. Process control of plants based on conventional nitrification and denitrification is very similar to that of activated sludge systems, with which operators are familiar. Nitrification is achieved by ensuring a minimum DO (Dissolve Oxygen) within the bioreactor and sufficient alkalinity and biomass within the bioreactor. Denitrification is achieved by ensuring sufficient carbon source is available. Process monitoring uses instrumentation familiar to WwTW operatives, deviation in process control (for example due to poor SRT (Solids Residence Time) or DO control)) will manifest itself in reduced process efficiency, however it is unlikely to cause process failure and stability can be recovered relatively quickly. Nitritation/denitritation relies on the suppression of oxidation of nitrite by NOB (Nitrite Oxidising Bacteria) by maintaining suitable SRT (equal to hydraulic retention time if a chemostat bioreactor is used) and elevated bioreactor temperature and low DO. Whilst in principle this process control is simple, in practice monitoring of nitrite, nitrate, Amm.N and DO in addition to temperature and ph in the bioreactors are all required to ensure process stability is maintained. Intermittent aeration can be applied if it is difficult to maintain the required hydraulic retention time in the bioreactor. If process control is not properly maintained (for example, due to poor instrument calibration or diagnosis of potential causes of instability) the process can fail completely and requires specialist input to reestablish it. For Anammox based processes, the balance between AOB and Anammox biological activity is required in addition to the suppression of oxidation of nitrite. The need for different SRTs for the slower growing

10 Anammox and faster growing AOB and NOB requires careful process control. Several approaches are adopted, all require the careful monitoring of ph, DO, nitrite, nitrate and biomass ratios. As with nitritation/denitritation, failure to maintain instruments, or diagnose process instability, in a timely manner can result in complete process failure. For Anammox processes, there is a further process risk in that the slow growth of Anammox microorganisms can delay process recovery when compared to other processes, hence availability of seed Anammox biomass is important. Furthermore, in Anammox based processes, heterotrophic denitrifiers can outcompete the autotrophic Anammox bacteria for nitrite but this can be prevented if biodegradable COD is kept at a low level. As the degree of process control and monitoring required to maintain process stability increases so does the risk of process failure. Careful consideration of this risk, in particular in relation to Anammox, is required when selecting the appropriate LTP technology. Mitigation of this risk is available by ensuring calibration of process control instrumentation and oversight by appropriate process expertise. High rate conventional processes as well as nitritation/denitritation and Anammox based technologies require reliable control of temperature in the bioreactors. The heat requirement will, in part, be satisfied by the exothermic nature of the reactions, however additional heat input and associated equipment will also be required. At the STC discussed in this paper excess heat is available from the THP, however careful consideration of the heat balance is important as the majority of high-grade heat is utilised in energy recovery within the THP process. Costs Operational (Opex) costs are strongly affected by the demand for consumables, such as chemicals and power. All conventional processes considered require addition of organic carbon to carry out denitrification and as the degree of alkalinity recovered during this process is usually insufficient to achieve nitrification, addition of caustic soda is also needed. The same applies to nitritation/denitritation processes except that alkalinity demand is satisfied by recovering it during the denitritation stage, enhanced by organic carbon addition. Demand for organic carbon is lower than for conventional processes. Oxygen demand for the latter process is expected to be up to 25% lower than for conventional nitrification/denitrification. In contrast, the Anammox based processes do not require addition of chemicals (if Amm.N removal lower that 80% is acceptable, which is the case in this project) and oxygen is supplied to partially oxidise only about 50% of ammonia to nitrites whilst the Anammox stage is an anaerobic process. Hence, it is expected that for Anammox based technologies, costs of consumables will be significantly lower than for other technologies. Other operational expenses to be considered are specialist process engineering input and maintenance. The amount of time required from process specialists for monitoring the process and adjusting control parameters is closely related to the process stability and robustness discussed above. The time spent is anticipated to be greater for nitritation/denitritation than conventional nitrification/denitrification, and increases further for Anammox based technologies. These costs can be significant and an important consideration in process selection, in particular for smaller plants where process specialist support is less readily available. The key components of the Capex of the LTP technologies under consideration are the capacity of bioreactors, blowers, chemical dosing and requirement for biomass recycling (i.e. clarifier). The Capex for this equipment is generally higher for nitrification/denitrification processes, decreases for nitritation/denitritation and is lowest for Anammox based technologies. An important component of both Opex and Capex that can be overlooked when selecting an LTP technology is the heat input required to maintain bioreactor temperature. Although, in principle,

11 adequate heat may be available from THP this may be low-grade heat and supplementary heating and/or tank insulation will be required. Maintenance costs are closely related to the complexity and capacity of the installed equipment discussed above. Maintenance costs will, therefore, be greater for nitrification/denitrification than for nitritation/denitritation and are lowest for Anammox based technologies. The Intellectual property (IP) costs for the technologies is a component of Capex. The proportion of IP cost in the Capex for each technology is not readily available and will vary on a project to project basis. In general, IP costs increase for proprietary technologies, in particular the relatively novel nitritation/denitritation and Anammox based technologies. The importance of Opex in the assessment of the Whole Life Cost (WLC) cannot be understated. For the technologies under consideration in this paper, the Capex, Opex and WLC ratio over a 40 year operating life was estimated. Table 2: Capex, Opex and WLC Comparison 1 Technology Capex Opex WLC Nitrification/Denitrification Nitrification/Denitrification (high rate) Nitritation/Denitritation Nitritation and Anammox Values normalised to a ratio of the lowest Capex value Table 2 demonstrates the variation in WLC between technologies is largely due to the variation in Opex, the Capex being broadly similar. Corresponding to the reduction in Opex is an increase in risk. Other Factors Other factors to be considered in selecting an LTP technology that were not applicable to the project examined in this paper are the implications of a Total Nitrogen (TN) consent and where P recovery is included as part of the process. Each of the LTP technologies under consideration denitrify to some extent and will reduce the TN within the LTP effluent. The extent of denitrification is greatest for nitritation/denitritation, although at the expense of addition of a carbon source. Anammox based processes do not completely denitrify, however no carbon source is required. Nitrification/denitrification processes denitrify to an extent determined by the available carbon and alkalinity. As discussed in previous sections, the management of phosphorus in liquors presents challenges. In the project discussed in this paper soluble phosphorus is precipitated and removed with the dewatered sludge. Where P recovery is required, there is an opportunity for integration into the LTP where ammonia is available for struvite precipitation. Proprietary processes incorporating both nitritation and Anammox and P recovery as struvite are available.

12 LTP Option Comparison and Discussion Overall comparison of the technologies is shown in Table 3, whilst a system of scoring was used for more detailed assessment, as explained further. Table 3: Liquor Treatment Technologies Overview Option CAPEX OPEX (year 1) WLC (40 year) Reliance - process recovery Process sensitivityoperator input Track record for sludge liquors Activated Sludge good poor poor good good good High rate activated Sludge good medium poor good good good Nitritation/denitritation good medium medium medium medium medium Anammox based good good good medium medium medium Once compliance with tender specification was deemed to be satisfactory for each option, the following additional criteria were used to assist the Client with selecting the preferred option: 1. General confidence in the proposed process including successful track record for treatment of sludge liquors in general and post THP liquors in particular 2. Allowance for specifics of THP liquors, such as recognition of presence of high levels of Amm.N, hard COD and inhibitory compounds in the liquors that may affect biological processes used in LTP 3. Process stability/robustness and operational flexibility to ensure that: the LTP can work under the anticipated variation of flows and loads; is capable of recovering from process failures; as well as maintain temperature in bioreactors at a suitable level, where needed. 4. Simplicity of LTP configuration and resilience 5. Operability of process (control requirements, ease of operation, familiarity to operators, reliance on highly skilled operators) 6. Ease of process commissioning/start-up 7. Footprint 8. Costs capex 9. Costs - opex 10. Environmental impact including carbon footprint 11. Additional benefits such as ease of incorporation of P removal if needed, possible source of revenue, beneficial impact on alkalinity balance Each criterion was assessed using a score selected from a range: 1 (poor), 2 (average), 3 (good), 4 (very good) see columns 3 and 4 in Table 4. The importance of each criterion has been determined by allocating a weight value (column 5). The weight value was then applied to the initial scores giving a weighted score for each criterion (columns 7 and 8) and a total score for each technology was determined. Table 4 illustrates, without being specific to any particular technology, the way the final score for each technology was determined.

13 Technology A Technology B Weighting/ Importance Balance of weights Technology A Technology B Table 4: Example of Liquor Treatment Technologies Comparison Scores Weighted scores Evaluation Criteria 1 General confidence in the proposed LTP % Allowance for specifics of THP liquors and other parameters in LTP design % Process robustness and operational flexibility % Simplicity of LTP configuration and resilience % Operability of process % Ease of process commissioning/start up % Footprint % Costs - Capex % Costs - Opex % Environmental Impact % Additional benefits % Total scores % Ranking 2 1 Note: Score ranking 1:Poor, 2:Average, 3: Good, 4: Very Good Summary The paper discusses risks associated with the conversion of WwTW to the main STC. It emphasizes the importance of the sludge and liquors characterisation for the selection of the LTP. The methodology which considers generic as well as site specific factors for the selection of the LTP has been developed in close consultation with DCWW. It resulted in the selection of an Anammox based process for the LTP. This decision was based on the balance of risks in the context of this STC versus the substantial WLC benefit of this technology. The risk management strategy included ensuring suitably qualified process expertise are included within the STC operational team. Due to uncertainties related to the P loadings, the aerated post-digestion tank will be fitted with a conical shaped bottom. If phosphorus loads prove to be higher than anticipated then MgO, as Mg source, will be dosed into the post-digestion tank which will promote struvite formation as well as increase of alkalinity in the liquors. The conical bottom will enable removal of struvite. Acknowledgements This paper would not have happened without the strong support of Dŵr Cymru Welsh Water. We are thankful to our colleagues who provided expertise that greatly assisted the research.

14 We would also like to express our appreciation to Victoria Wilson for sharing her knowledge and experience with us during the course of the project. References Bott C. (2016) personal communication Bungay S., Chapman K., Buchanan A., Crassweller J., Barnes L., Smyth M. (2007) Fate of Phosphorus during Anaerobic Digestion of Sludge from an EBPR plant. In Proceedings of the 12th European Biosolids and Organic Resources Conference, Manchester. Figdore B., Wett B., Hell M., Murthy S. (2011) Deammonification of Dewatering Sidestream from Thermal Hydrolysis-Mesophilic Anaerobic Digestion Process In Proceedings of the WEF, Nutrient Recovery and Management. Hellinga C., van Loosdrecht M. C. M. and Heijnen J. J. (1998). The Sharon process: an innovative method for nitrogen removal from ammonium-rich waste water. Water Science and Technology, 37(9), Jardin N., Thöle D. and Wett B. (2006). Treatment of sludge return liquors: experiences from the operation of full-scale plants. In: Proceedings of the Water Environment Federation, WEFTEC 2006, pp