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Aeration Design Aspects for Aerobic Digestion of Mechanically Thickened

Waste Activated Sludge

Arnim Hertle, GHD, ahertle@ghd.com.au Derek de Waal, GHD, ddewaal@ghd.com.au EXECUTIVE SUMMARY A large dairy factory in Victoria was frequently experiencing strong odour emissions from their wastewater treatment plant with mounting pressure from the community and EPA to control and eliminate these odours. The odours were associated with the sludge management practices at the site. The investigations focussed on oxygen transfer and aeration capacity of the existing 3 No aerobic digesters operating in series. Because of the high design dry solids concentration in the digesters of 4 to 5% DS and the use of mechanical sludge thickening with polymer dosing, usual aeration system design parameters for oxygen transfer do not apply anymore. Effects of the sludge viscosity on the oxygen transfer have to be taken into account. For 4% DS, the oxygen transfer rate was calculated as only half the already low value allowed in design and significantly reduced (to about one tenth) compared to normal activated sludge applications. Based on the results from laboratory digestion trials at various DS levels and published data from aeration systems working at DS contents similar to those in the digesters, a model was set up that took into account DS and viscosity related impacts on the aeration system efficiency. The model was calibrated using observed digestion plant operating parameters and performance data. The calibrated model then allowed reviewing of the existing installation and evaluation of options for the augmentation of the digestion plant. A major upgrade of the aeration system in the existing 3 digesters would have been required in order to compensate for the significantly reduced oxygen transfer rate and the fact that the aeration system was generally undersized. Further, the installation of strong mixers would have been required to improve the oxygen transfer in this sludge and thereby achieve a robust performance with minimum odour risk. It was therefore decided to install a fourth digester and operate the digestion plant at only 2% DS. As oxygen transfer into the mechanically thickened sludge at this concentration is already fairly similar to that in mixed liquor, this option resulted in only minor aeration system upgrade requirements and no additional mixing to achieve satisfactory aerobic conditions and a sufficient aerobic sludge age for good sludge stabilisation.

INTRODUCTION A large dairy factory in Victoria was frequently experiencing strong odour emissions from their wastewater treatment plant (WWTP) with mounting pressure from the community and EPA to control and eliminate these odours. The WWTP is exclusively treating the industrial milk processing wastewater; all human wastes are excluded from this system. The plant is a trickling filter / activated sludge plant operating in series with excellent performance in BOD removal as well as biological nitrogen and phosphorus removal. The excess activated sludge from the plant is mechanically thickened with a gravity drainage deck, treated by means of aerobic digestion and then dried in sludge drying beds (SDBs). The scope of the project was to identify the main odour source and to recommend a solution to this problem. IDENTIFICATION OF SOURCE AND CAUSE OF THE ODOURS Several inspections of the wastewater treatment plant and sludge treatment areas were undertaken together with the operations personnel. Most of these site visits were directly after the client received odour complaints from the community. Odours were present around the balance tanks where the untreated wastewater with its high BOD content, is stored temporarily. However, it was found that these odours were localised only. Furthermore, the odour emissions from the wastewater treatment process were negligible. Therefore both these sources were not considered relevant for the odour complaints. However, some odour emissions of significant strength were noted from the 3 No aerobic digesters when the aeration was turned off and on again and pockets of strong odours were found at the SDBs. The original WWTP design required the waste activated sludge (WAS) to be thickened to approximately 3.5% to 4% dry solids (DS) prior to the digestion process. It was initially assumed that this relatively high sludge concentration was indirectly the cause for the odour emissions because of: - Insufficient degree of stabilisation of the sludge in the aerobic digesters; and - Ongoing biological degradation of the only partially stabilised sludge in the SDBs while

the sludge was being dried. Therefore, as a preliminary measure, the digestion process was operated at a lower solids concentration of around 2.8% DS, instead of 4% DS as per design, over a 12 month period. While improved stabilisation was achieved with lesser odour potential, this practice still led to unsatisfactory stabilisation results because of the insufficient hydraulic detention time in the sludge digestion system. Based on these preliminary outcomes, it was agreed that a process design review was required to identify the scope of works required to upgrade the existing aerobic digestion system to produce well stabilised WAS.

DIGESTION PROCESS REVIEW Digestion process objectives The general process objective of most digestion systems is sludge stabilisation. By definition sludge stabilisation involves the reduction of the potential for odour generation from the sludge during extended storage periods after treatment. Odour generation during storage is commonly due to anaerobic degradation processes that produce odorous compounds, which are released as gases when either their solubility limit is reached and/or the sludge is disturbed. Stabilisation removes this odour potential by removing most of the (relatively) easily biodegradable substrate in a dedicated digestion process thereby limiting further biological activity, prior to sludge storage. The degree to which any sludge needs to be stabilised before it can be stored safely is to some extent dependent on its intended storage time. The storage of wet sludge has a greater odour potential than dried sludge considering that biological activity is more viable under such conditions. For this project, the digested sludge is pumped to SDBs where the sludge is air dried through the combined evaporative effects of the sun and wind. The sludge is stored in a wetted state for typically 3 to 4 months (but up to 12 months) before it is sufficiently dry to be removed from the beds and stockpiled. This long storage period requires the digested sludge to be well stabilised when discharged to the sludge drying beds in order to avoid anaerobic degradation during storage and consequent odour generation. Digester operating temperature The operating temperature of the digesters was reported to be between 25 C and 35 C. These temperatures characterise a mesophilic temperature range which is relatively unusual for typical aerobic sludge digestion processes. This higher operating temperature is due to a combination of the generally higher temperature of the factory effluent (relative to typical municipal applications), mild climatic conditions, and the autogenous (self) heating of the aerobic digestion process. Loads A review of the solids load showed that the digestion plant was overloaded by approximately 40% (ca. 1,550 kg DS/d instead of 1,090 kg DS/d). Hydraulically the digestion plant was also significantly overloaded. It was designed for a sludge feed of 1,090 kg/d at 4-5% DS whereas in practice the solids load was higher and the sludge feed concentration was only 2.8% DS. The problems with the insufficient hydraulic retention time were aggravated when an attempt was made to increase the oxygen transfer into the digestion process by installing additional surface aerators. In order to avoid splashing by these, the sludge levels in the respective digesters were lowered, which led to a further decrease in hydraulic retention time. In addition, the WWTP was operating at a lower sludge age than assumed in the original design calculations. As a result the digesters were fed with lesser stabilised sludge which in turn requires a longer sludge retention time in the digestion process.

Review of existing performance An evaluation of sludge analysis results showed an average volatile suspended solids (VSS) destruction of only around 7.4%. It is noted that a low VSS destruction rate can theoretically still lead to a well stabilised sludge with low odour emission potential depending on the biological availability of the organic matter, in other words depending on how much of the total volatile solids are digestable. However, neither the proportion of digestable VSS in the sludge nor a relationship between VS destruction and odour emission potential had been established. The original WWTP design calculation assumed 30% VSS destruction in the digestion process. Whilst this expected level of performance is only an assumed measure for satisfactory stabilisation whereby sludge has a low odour potential, it was noted that the existing sludge digestion process did not meet this performance expectation. Comparison with other reported digester performance Little suitable operating data from other aerobic digesters operating with mechanically thickened WAS at 2.8% DS and above was found. However, Grohmann and Ppel (1993) investigated the impacts of different aeration and mixing systems on VSS destruction and other performance parameters for aerobic thermophilic digesters operating on mechanically thickened mixed raw primary and secondary sludge of 3 to 5% DS in large pilot / small technical scale. The aeration systems that were tested included fine bubble diffused air aeration, however, assisted by a mechanical mixer. They found that at the high DS concentrations fine bubble diffused air aeration systems required that additional element of mechanical mixing in order to perform. The VSS destruction rates achieved in those tests was around 30%. A comparison of the operating parameters of their test set up with the operating parameters of the dairy factory digesters showed, that the dairy factory plant had a significantly longer sludge retention time, but significantly lower total power inputs and aeration power inputs. Installation of approximately 40 kW mixing per tank would be required to provide mixing energy similar to the set up used by Grohmann and Ppel. Laboratory digestion tests In parallel with the theoretical review of the load parameters, laboratory tests were carried out with fresh thickened excess activated sludge with 12 days aerobic sludge age and a concentration of approximately 3% DS. The sludge sample was taken from the feed to the digesters. It was split into three sub-samples and two of these sub-samples were diluted down to 2.5% DS and 2% DS. All three samples were then aerated and observed for 50 days. Small samples were regularly taken and analysed and water losses due to sampling and evaporation were compensated for by addition of demineralised water. The laboratory trials showed a high rate of biodegradation, measured as volatile suspended solids (VSS) destruction, during the first 26 days of sludge aeration for all three samples. After that period, the rate of VSS destruction slowed down, but was still considerable. The graph resembles the shape of a growth phase followed by an exhaustion phase. However, for reasons of simplicity it was approximated with 3 linear functions where the second function is considerably steeper than the first and the third. This is shown in Figure 1. It is noted that the graph for 3% DS shows a delay of approximately 5 days until this sludge also enters into the second, more rapid VSS destruction phase. This could be due to aeration problems in the higher concentrated sludge, as explained later.

In practice, operation of the digesters at 2.8% DS had already resulted in an improved sludge stabilisation compared to operation at the design DS concentration of 4% DS, despite the shorter solids retention time (SRT) in the digesters. As the laboratory trial showed that 2.8% DS again had a slower VSS destruction performance than the even thinner sludges, it was considered likely that impacts of the solids concentration on the aeration system performance were a main reason for the under-performance of the sludge stabilisation plant. A further review of the aeration system was therefore carried out.

Figure 1 VSS destruction during the laboratory sludge aeration trials AERATION SYSTEM DESIGN REVIEW Existing system The 3 No open digester tanks are similar in capacity and design and are operated in series. Aeration is provided by two blowers which supply air to fine bubble membrane diffusers mounted on the floor of the digesters. The diffusers are fitted with silicon membranes. In addition to the fine bubble diffused air aeration system, surface aerators were installed in the first and second digester in an attempt to increase the oxygenation of the sludge. However, both were of small nominal drive power. It was observed that the sludge in the first digester was very thick. The pattern that the aeration air bubbles produced on the surface of the sludge was very atypical for fine bubble diffused air aeration as only a number of very large bubbles appeared on the surface indicating coalescing of the fine bubbles. The throw pattern from the surface aerator and the induced flow pattern also indicated a high viscosity, presumably due to high solids content and polymer addition from the WAS thickening process. Theoretical background The aeration of sludges is very often carried out with mechanical aeration devices such as aspirating aerators/mixers or surface aerators. For those combinations the aeration

system design is based on the actual oxygen demand (AOR) of the digestion process which is then converted into an energy demand, based on typical aeration efficiencies (kg O2/kWh). Safety factors and factors that account for the particular properties of sludge, for example a higher viscosity compared with water, are included in the aeration efficiencies. Where fine bubble diffused air aeration is employed for the digestion process, a different approach similar to that used for the design of diffused air aeration systems for activated sludge processes should be used. It is more difficult to establish typical aeration efficiencies for diffused air aeration systems than for mechanical surface aeration systems due to the smaller number of such installations in operation. In addition, diffused air aeration provides mixing only by means of the rising air bubbles. However, good mixing, which can only be achieved by mechanical means, is of increasing significance for the oxygen transfer the thicker the sludge is. Conversely, power inputs for a diffused air aeration system becomes less significant for thicker sludges because power is directly proportional to sludge depth and air flow rate, and not the oxygen transfer rate. The calculation of the aeration efficiency of diffused air aeration in sludge applications therefore has to include the power for mechanical mixing. Little data has been published on the digestion of mechanically thickened activated sludge using diffused air that would allow a typical aeration system design calculation. However, laboratory, pilot and full scale membrane bioreactor test results had been published that particularly dealt with fine bubble diffused air aeration systems in those reactors (Gnder, 1999, Wagner et. al., 2001). As membrane bioreactors can operate at activated sludge concentrations significantly higher than conventional activated sludge systems, these results were considered relevant for the aeration system design for the dairy factory digesters. In particular the laboratory and pilot scale work (Gnder, 1999) investigated the effects of the increased viscosity at higher activated sludge concentrations on the factor. The tests were carried out up to sludge concentrations similar to those in the digesters. It was found that the DS concentration as well as the viscosity of the mixed liquor had a significant impact on the factor such that it decreased significantly with increasing DS concentration and viscosity. These findings were converted into formulae that allow estimation of as a function of DS or viscosity. It is noted that due to the non-Newtonian characteristics of mixed liquor, in particular at higher DS levels, its viscosity can only be determined as dynamic viscosity. For non-Newtonian fluids viscosity is a function of the shear rate. Therefore, a representative shear rate had to be determined that best characterised the relevant processes at the air bubble, where the oxygen transfer from the air into the mixed liquor (or sludge) actually takes place. Gnder used a shear rate of 40 s-1, based on typical values for steady state bubble rise velocity and bubble diameter, to describe the shear forces in aerated mixed liquor. The full scale tests carried out by Wagner et. al. (2001) confirmed the impact of the mixed liquor suspended solids content on . The factors measured in full scale were higher than the ones obtained by Gnder which was attributed by Wagner to the difficulties in transferring results from small scale test plants to full scale plants.

Methodology Based on the air throughputs of the blowers and the characteristics of the diffusers, the Standard Oxygen Transfer Rate (SOTR) of the aeration system in each digester was calculated. For the digestion process the total actual oxygen requirement (AOR) was estimated based on the observed VSS destruction and a specific oxygen requirement per kg VSS destroyed. In the original design calculations for the factory WWTP the latter had been assumed as 2.3 kg O2/kg VSS destroyed. This is far higher than for example the 1.4 kg/kg VSS destroyed that Eckenfelder (1980) suggests. Unfortunately it was not documented why and how the original design calculation arrived at the higher specific oxygen requirement. Two parallel calculations were therefore carried out with both factors. For the purpose of the work on the dairy factory digesters Gnders equations were used to estimate for various sludge DS concentrations. A formula, developed by comparing Gnders and Wagners results for similar DS concentrations, was then applied to the results to transfer them from lab / pilot scale to full scale. The thus calculated factor was used, together with other factors such as the salinity factor etc., like in a normal aeration system design calculation, to convert the calculated AOR to an equivalent Standard Oxygen Requirement (SOR). In theory the SOTR calculated from the aeration system properties and the SOR calculated from the observed VSS destruction should approximately match. However, the results showed a significant difference. The main reason for this difference is considered to be an inaccurate estimate of because of: 1. Impacts of the polymer from the mechanical thickening process on the sludge

viscosity and / or oxygen transfer, in particular in the first digester as the polymer is expected to hydrolyse in the digester and thus lose its bridging effect on the sludge particles;

2. Different activated sludge properties due to the purely dairy-industrial nature of the wastewater; and

3. Inaccuracies in transferring the calculated factors from laboratory / pilot scale to full scale using a formula that was based on full scale test results at comparatively low DS concentrations only.

A correction factor CF was then introduced to modify (giving * = /CF) such that the respectively calculated SOR* matched SOTR. The impact of the two different specific oxygen requirements per kg VSS destroyed mentioned above was only a different CF. Using the correction factor obtained from the review of the observed digester performance, the aeration design parameters and oxygen requirements for design and current operating conditions were calculated.

Results The results from the review of observed digester performance are shown in Table 1. Table 1 Review of observed digester performance Parameter Unit Results for

2.3 kg O2/kg VSS(original design

calculations)

Results for 1.4 kg O2/kg VSS

(Eckenfelder)

SOTR kg/h 62 1) 62 1) VSS feed kg/d 1,290 1,290 VSS destruction % 7.4% 7.4% AOR kg/h 9.2 5.6 factor --- 0.23 0.23 CF --- 1.5 2.4 * --- 0.15 0.10 AOR:SOR* --- 0.14 1) 0.09 1) SOR* kg/h 62 62 1) Existing installation with digesters operating at reduced level. The Table shows the results for both VSS-destruction-specific oxygen requirements as mentioned above, to demonstrate that the end result is the same because differences in intermediate results of the calculus are compensated for in the correction factor CF. With the correction factor obtained from the assessment of the observed digester performance, the aeration parameters and oxygen requirements for design VSS loads and current VSS loads to the digesters were calculated as shown in Table 2 and Table 3. The calculations were carried out over a range of DS concentrations in the sludge to allow an assessment of the positive effect of lower sludge concentration on the aeration system requirements versus its negative effects on the required digester volume. Table 2 Aeration parameters and oxygen requirements for design digester load at various DS Parameter Unit 2.0% DS 2.8% DS 4.0% DS SOTR kg/h 140 1) 140 1) 140 1) VSS feed kg/d 860 860 860 VSS destruction % 30% 30% 30% AOR 2) kg/h 24.7 24.7 24.7 factor --- 0.35 0.23 0.11 CF --- 1.5 1.5 1.5 * --- 0.23 0.15 0.07 AOR:SOR* --- 0.21 1) 0.14 1) 0.06 1) SOR* kg/h 119 181 378 1) Existing installation with all digesters operating at maximum level. 2) For 2.3 kg O2/kg VS destroyed It is evident from Table 2 and Table 3 that the existing aeration system was undersized for the design duty and even more so for the current duty.

Table 3 Aeration parameters and oxygen requirements for existing digester load at various DS Parameter Unit 2.0% DS 2.8% DS 4.0% DS SOTR kg/h 140 1) 140 1) 140 1) VSS feed kg/d 1,290 1,290 1,290 VSS destruction % 30% 30% 30% AOR 2) kg/h 37.1 37.1 37.1 factor --- 0.35 0.23 0.11 CF --- 1.5 1.5 1.5 * --- 0.23 0.15 0.07 AOR:SOR* --- 0.21 1) 0.14 1) 0.06 1) SOR* kg/h 178 271 567 1) Existing installation with all digesters operating at maximum level. 2) For 2.3 kg O2/kg VS destroyed As a result of a whole of life costs analysis it was concluded that the most cost effective augmentation of the digestion plant would be achieved by the addition of a 4th digester in series plus operation at 2% DS feed sludge concentration. When it became clear that a combination of additional digester volume and reduced feed DS content was likely to be the preferred augmentation strategy, the surface aerators in the first two digesters were not included in the calculation of SOTR in order to avoid too much conservatism in the design calculations. The reason behind that was that the correction factor obtained from the observed digester performance is an average over the three digesters. However some factors such as the impact of the polymer or a higher fat content in the sludge would decrease once a fourth digester is added and the plant is operating at a lower overall DS level. It was therefore concluded that, within the general accuracy of the calculus, which includes many assumptions and estimates, the correction factor was likely to already be on the conservative side for the augmented digestion plant. Inclusion of the surface aerators would have increased it further and thus probably lead to an oversized system. It was concluded that, if required in future, simple pulley changes on the blowers could increased air throughput. CONCLUSION An assessment of the aerobic digestion process of a dairy factory WWTP and a detailed review of the aeration system design together with operational evidence and the results of laboratory sludge digestion tests, showed significant impacts of the DS concentration on the aeration system efficiency, most likely due to the increasing viscosity of the sludge with increasing DS levels. These findings are in line with results from aeration trials carried out on membrane bioreactors. The predictions of theoretical aeration calculations were calibrated against observed digester performance. These results and the results from the laboratory test enabled the formulation of a model for the digestion process and aeration system which was then used to fully assess the existing installation and determine the most cost effective augmentation strategy.

It was found that the digester aeration system was undersized and could not deliver sufficient oxygen to provide truly aerobic conditions. Key issues were - a shorter aerobic sludge age in the WWTP and a significantly higher VSS feed load

due to higher WAS production than assumed in the design; - that the oxygen transfer rate was significantly lower than assumed in the design

calculations; - that the installed aeration capacity was significantly less than the design capacity; and - that as a result the total aerobic sludge age was not long enough to provide

stabilisation. As a result of a whole of life costs analysis it was concluded that the most cost effective augmentation of the digestion plant was the addition of a 4th digester plus operation at 2% DS feed sludge concentration. REFERENCES Eckenfelder W. W., jr. (1980); Principles of water quality management; CBI Publishing Company Inc.; Boston, Massachusetts; p. 437 Grohmann W., Ppel H. J. (1993); Aerob-thermophile Klrschlammstabilisierung - Verbesserte Zukunftsaussichten durch optimierte Mischung und Belftung?; Verein zur Frderung des Instituts fr Wasserversorgung, Abwasserbeseitigung und Raumplanung der Technischen Hochschule Darmstadt; Darmstadt; Schriftenreihe WAR 66: 31. Darmstdter Seminar Abwassertechnik: Klrschlammbehandlung und Klrschlammentsorgung Stand und Entwicklungstendenzen; pp. 67-98 Gnder B. (1999); Rheologische Eigenschaften von belebten Schlmmen und deren Einfluss auf die Sauerstoffzufuhr; Korrespondenz Abwasser 46 Nr. 12; pp. 1896-1904 Wagner M., Cornel P., Krause S. (2001), Sauerstoffeintrag und -Werte in Membranbelebungsanlagen, Korrespondenz Abwasser 48 Nr. 11; pp. 1573-1579