Pretorius, Coenraad, Pretorius

Pretorius, Coenraad

Coenraad is GHDs service line leader for wastewater treatment and recycling in the Americas. He has over 25 years of experience in process design for water and resource recovery. Recently he has focused on process control and energy saving strategies, including optimization of mixing and aeration in aeration basins as well as mixing in anaerobic digesters.

Titles from this speaker

Alternate: Thermal Hydrolysis for All: Options for Small and Medium Size Plants

Abstract

Anaerobic digestion of particulate substrates like municipal sludge is a multi-step biological process that begins with hydrolysis: the breaking down of larger macromolecules into small molecules amenable to fermentation. As seen in Figure 1, hydrolysis is the rate-limiting step for sludge digestion and this recognition has resulted in the creation of many different technologies over the last 20 years to try to speed up the hydrolysis process. Development of these technologies has generally been pursued to maximize the efficiency of existing anaerobic digestion assets including increasing volatile solids (VS) destruction and biogas generation. Of these technologies, thermal hydrolysis has had staying power and become the 'de facto' sludge pretreatment alternative most commonly proposed for digestion process intensification. As its name implies, the thermal hydrolysis process (THP) enhances hydrolysis by treating wastewater solids with high temperature. An established THP technology applied in North America, thermo-pressure, utilizes a combination of high temperature and high pressure to facilitate the breakdown of sludge solids. The typical advantages of this THP include intensification of digester capacity typically two-fold, i.e., doubling the allowed VS loading rate, reduced sludge viscosity facilitating digestion at much higher total solids (TS) concentrations, higher VS destruction and biogas yield, improved dewatering resulting in higher cake dry solids content and less wet biosolids cake product transportation, as well as a high-quality end-product for beneficial reuse. The principal disadvantages of this THP typically cited include operational complexity, specialized staff technical knowledge and skills to operate and maintain the high temperature and pressure systems as well as the increased ammonia recycle loads associated with higher VS destruction. The discussion above assumes that THP is used as pretreatment for anaerobic digestion, but as shown in Figure 2, there are at least four different process configurations for using THP. It may be used for hydrolysis of both primary and waste activated sludge (WAS) labeled Pre-Digestion All in the figure. To maximize the benefit per dollar spent on capital as well as operating and maintenance cost, THP may be used only to improve hydrolysis of the more difficult to digest WAS, labeled as Pre-Digestion WAS-only. THP may also be used to treat digestate the benefits of doing so include achieving high cake solids concentrations in subsequent dewatering and producing a sterile product. The filtrate or centrate, containing the bulk of the solubilized organics, may be recycled to the digester to increase biogas production and reduce the organic load recycled to the liquid treatment plant. This is labeled Post-Digestion. One may also consider intermediate THP, that is feed digestate to THP prior to further digestion, as another strategy to reduce the cost of implementing THP. Finally, one may feed dewatered sludge to the THP so that THP is used to improve solids quality prior to disposal, regardless of the level of treatment prior to THP, labeled Post-Dewatering. The application of THP to date has been limited to larger plant sizes as shown in Figure 3. Most of the THP facilities in North America are at large facilities such as Blue Plains (Washington, DC), TRA CRWS (Dallas, TX), Bonnybrook (Calgary) and others. In the last several years smaller package THP units have been introduced to the North American market for medium size plants, of which there are two installations at plants rated 60 ML/d or 15 mgd (Franklin, TN) and 113 ML/d or 30 mgd (Pontiac, MI), which are still relatively large the majority of plants throughout North America are much smaller. An alternative to thermo-pressure is thermo-alkaline hydrolysis which uses elevated temperature and pH for hydrolysis and is a technology that can be readily applied at small and medium size plants. This type of THP addresses the complexity and high temperature/pressure issues by using lower processing temperatures and ambient pressure with the tradeoff of adding an alkali chemical. It can be used in either a Pre-Digestion or a Post-Dewatering configuration. In other words, thermo-alkaline is also an option for facilities that do not have any form of sludge stabilization. There are 13 full-scale facilities of this technology operating in North America since 2006. Most of those facilities use the Post-Dewatering configuration to create Class A quality high-solids liquid fertilizers. Operation of thermal-alkaline hydrolysis in one of the Pre-Digestion configurations is a more recent direction for this technology in North America. Pre-Digestion has been more commonly utilized in Europe. Lystek is operating at several small plants rated 5 to 15 ML/d (1.3 4.0 mgd) and several medium size plants rated 27 to 50 ML/d (7 14 mgd); some of these recycle a portion of the thermally hydrolyzed product back to anaerobic digestion for additional solids destruction and biogas generation. The technology is also used at the regional 400 ton/day Organics Material Recovery Facility at the Fairfield-Suisan Sewer District plant in Fairfield, CA. Lystek is currently developing the Pre-Digestion configuration. Pondus has one installation in North America at a plant rated 106 ML/d (28 mgd) in a Pre-Digestion configuration, with more than ten similar systems installed in Europe at facilities as small as 38 ML/d or 10 mgd. THP for small and medium size plants is arguably an even better value proposition than at large plants because smaller facilities often dispose of biosolids in liquid form and may have inefficient, or even no, digestion and/or dewatering processes. THP can improve product quality and reduce viscosity to improve disposal of liquid biosolids, allowing direct subsurface application on arable land, so that there is no need for the farmer to store the product prior to land application. Table 1 compares the thermal-alkaline hydrolysis (from both Lystek and Pondus suppliers) with more widely known thermo-pressure hydrolysis processes (from Cambi, and other suppliers). The difference in complexity and operating conditions are included. Note than Pondus is the only technology that treats solids at typical thickened solids concentrations as commonly used for anaerobic digester feed. The Lystek system uses potassium hydroxide to improve the fertilizer value of the product. The thermo-alkaline processes can be used to provide substrate for biological nutrient removal systems. Thermo-pressure and the Pondus system are typically used to improve anaerobic digestion performance, in other words to increase VS destruction so that biogas production is increased and the mass of residual biosolids is reduced. The Lystek technology, by comparison, has historically been used to convert residual biosolids into a marketable fertilizer product and only more recently starting to be applied as pretreatment to improve anaerobic digestion. The goals of THP may be different at different size plants. At large, urban plants the focus may be to maximize energy recovery and minimize the mass of residuals needing disposal, while reducing the use of chemicals where possible. Thermo-pressure THP is well suited to this application. Large plants are also better equipped to meet the complexity and specialized staffing requirements of thermo-pressure THP. The increased recycle ammonia load may be addressed by implementing another specialized sidestream treatment technology also feasible at a large plant. At smaller and especially rural plants, the ability of some thermo-alkaline THP to convert partially or unstabilized sludge into a marketable fertilizer product would be prized. Improving hydrolysis is of limited value to a plant that does not implement anaerobic digestion or where the biogas from anaerobic digestion is flared because the flow is too low to make cogen or other energy recovery options viable. However, with higher prices of natural gas, sale price premiums for renewable natural gas, and small cogen units like microturbines, implementing anaerobic digestion with thermo-alkaline hydrolysis at small plants is becoming increasingly feasible. Implementation of THP at smaller facilities also opens the opportunity for public-private-partnerships to reduce risks and securing private equity, or public-public-partnerships to gain economy of scale and overcome challenges due to skilled labor and part-time dewatering operations. Thermo-alkaline THP is less complex and operates at lower temperature and pressure conditions than thermo-pressure THP systems. In addition, Post-Dewatering configurations allow unstabilized solids to be treated by THP, so that the technology can be applied at plants without anaerobic, or any, digestion. That has allowed thermo-alkaline THP technology and its benefits to be implemented at plants as small as 5 ML/d or 1.3 mgd.

This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.

SpeakerPretorius, Coenraad

Presentation time

13:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionSession 15: New Research in Anaerobic Digestion

Session number15

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicThermal Processes and Biochar

Author(s)

C. Pretorius

Author(s)C. Pretorius1, J. Kraemer2, J. Surti3, 4,

Author affiliation(s)GHD1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158829

Volume / Issue

Content sourceResiduals and Biosolids

Word count13

Blower Turndown: How Low Can You Go? And Is It Low Enough?

Abstract

BACKGROUND
Our industry is moving towards more environment-friendly designs, taking up the challenge of net zero energy and carbon footprint reduction. It has been shown (SAIC, 2006) that aeration in the typical activated sludge plant consumes approximately 50% of the total power demand. Recently, we have seen the implementation of new blower technologies as well as new aeration control strategies focused on saving power. Some strategies, such as ammonia-based aeration control, strive to may reduce oxygen demand by allowing simultaneous nitrification-denitrification while also improving effluent quality. However, if the blower system has insufficient turndown to accommodate the full range of required airflows, these new strategies will not achieve the full potential savings. This paper investigates the common causes of overestimating minimum airflows and identifies the factors that need to be considered to more accurately determine the blower turndown requirements.
METHOD
When designing aeration systems, designers tend to make conservative assumptions, that maximize the estimated blower capacity. Most parameters are assumed to have constant values for a given design. However, some variables are subject to diurnal and/or seasonal variation: standard oxygen transfer efficiency (SOTE) for the diffusers, the alpha factor (alpha), mixed liquor and air temperatures. A dynamic process model for a biological nutrient removal plant treating 14 ML/d (3.7 mgd) was used to estimate the diurnal oxygen demand for a typical day. The required blower airflows were determined for the hottest day of the year, using hourly climatic data. The calculation was repeated for the coldest day of the year. Finally, the airflows for the cold day at startup was calculated, while adjusting alpha and SOTE. The impact of other design conditions, including maximum month, peak day, minimum day, etc. will be discussed in the manuscript. The SOTE for any fine bubble diffuser increases with reduced airflow and under reduced loads, as reflected in the curves from diffuser suppliers. SOTE depends on several factors, including diffuser submergence depth, layout and the allowable operating range for the selected diffusers. The designer needs to carefully adjust the number of diffusers to balance all these factors. Of all the aeration parameters, alpha is the least understood. It is surprising that it is so seldom measured, considering its importance to aeration system design and operation. Off-gas testing has been used for decades to determine the value of alpha, but only recently has it been measured over extended periods, so that diurnal variations in its value could be established. Trillo et al (2014) showed that in an oxidation ditch equipped with fine bubble diffusers, alpha varied by a factor of about 1.5 over a single 24-hour period (see Figure 1). Sosa Hernandez et al (2019) showed that during constant feed flow in a rectangular BNR aeration basin, alpha may vary by a factor of 2.0. Under typical diurnal load variations, excluding flow equalization, the change increased to a factor of 2.5. Using previously published data, Jiang et al (2017) developed a negative correlation between local COD concentration and alpha. Rosso and Stenstrom (2005) showed that alpha is a function of operating solids retention time, that it increases along the flow path in the aeration basin and that adding an unaerated selector upstream of the aerated zones increases alpha. This highlights the inadequacy of assigning a single value to alpha during design. Clearly, the need to assess the effect of variations in alpha will be critical in identifying the full range of airflows that a blower system will need to provide. The wastewater temperature will vary seasonally. At most plants it would be reasonable to assume that peak design OTR could coincide with maximum wastewater temperature and minimum design OTR with minimum wastewater temperatures. This would again tend to move the minimum and peak design airflows further apart. Many design engineers would take the analysis no further than the normalized or standardized airflows (Nm3/h or scfm). Blowers, however, are constant volume machines and the normalized or standardized capacity of a given blower changes with inlet air temperature and humidity. Using only normalized airflows implicitly assumes the blowers will always receive air at one temperature and humidity, typically the maximum design temperature and humidity used for determining the maximum blower capacity. For the purpose of illustration actual blower airflows are used in this paper. The minimum airflow to a specific zone may be set by mixing requirements. Using an overly conservative mixing airflow may result in the airflow to a given zone set by the mixing requirement for the bulk of a typical day.
RESULTS AND DISCUSSION
The impact of all these factors can be illustrated by an example. Figure 2 shows the actual blower airflow requirements. The figure shows that if a constant alpha value is used (orange line), the result would tend to underestimate peak flows while overestimating the minimum flow compared to variable alpha (gray line). Both lines are based on the hottest day in summer. When the coldest day in winter is considered (yellow line), the entire airflow is lower, so the minimum airflow is 14% lower for a location such as the northeastern part of North America where both hot summers and cold winters are the norm. The figure also illustrates the impact of startup conditions: while the startup flows and loads are 23% below those of design, the minimum airflow is 33% lower, due to higher alpha and SOTE values at startup. The minimum diurnal airflow required at startup is less than 20% of the maximum airflow, while the minimum oxygen demand is 33% below maximum. Using constant SOTE, alpha and air temperature would overestimate the minimum airflow by 73%! Airflow to aerated zones are sometimes maintained at a certain minimum flow to ensure mixing. The consequence of setting such a minimum is that the DO concentration would increase above the process setpoint when oxygen demands are low. For some systems, particularly those aiming to achieve enhanced nutrient removal, high DO might be more detrimental to the process than a small amount of settling. Pretorius et al. (2015) reported that an air flux 50% below the commonly accepted minimum mixing guideline (0.12 scfm/sf or 1.1 Nm3/m2.h) was sufficient. Anecdotal observations from other WRRFs support this observation, suggesting the guideline can be lowered and tailored for each application. Different blower technologies offer different turndown ratios, and the designer must consider these and the implications for the number of units required. The manuscript will include a full discussion of blower selection.
CONCLUSION
Our analysis shows that assuming constant values for certain variables can lead to a significant underestimation of the required downturn, leading designers to overestimate the actual minimum downturn by over 70%.

Blowers must be designed for the full range of operating conditions they may to encounter during their service lives. The traditional approach of using a single set of design parameters, tends to overestimate minimum blower airflow requirements. Using conservative, but opposite assumptions when estimating minimum and maximum airflows would assist designers in determining the true required blower system operating range and allow power savings associated with new technologies to be realized.

SpeakerPretorius, Coenraad

Presentation time

08:30:00

08:55:00

Session time

08:30:00

10:00:00

TopicAdvanced Level, Energy Production, Conservation, and Management, Municipal Wastewater Treatment Design

Author(s)

Pretorius, Coenraad

Author(s)Coenraad Pretorius1; Dale de Kretser2; Jeremy Kraemer3

Author affiliation(s)GHD, Irvine, CA1; GHD, Melbourne, VIC, Australia2 ; GHD, Waterloo, ON, Canada3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158490

Volume / Issue

Content sourceWEFTEC

Word count13

Description: Real-world Reliability and O&M of High-speed Turbo and Integrally-geared...

Real-world Reliability and O&M of High-speed Turbo and Integrally-geared Blowers

Abstract

Aeration blowers consume around half of all electricity used at a water resource recovery facility (WRRF). Choosing the most efficient blowers can save money, electricity, and greenhouse gas emissions. High-speed turbo blowers and single-stage integrally-geared (SSIG) blowers offer some of the highest efficiencies for process aeration, but operations and maintenance (O&M) staff may not have experience with these technologies. This paper documents comparative O&M experiences between turbo and SSIG blowers compiled from interviews with thirteen (13) North American water utilities with medium to large size blower installations as well as the authors’ own experience and past projects. In addition, turbo blower reliability and failure data and maintenance contract information were solicited from turbo blower manufacturers and summarized. This information was used to support one water utility’s blower technology decision and the authors hope this information will be useful to other utilities as well.

This paper compares O&M experiences with turbo blowers and single-stage integrally-geared blowers from interviews with 13 North American water utilities with medium to large blower installations and the authors' experience and projects. Turbo blower reliability data and maintenance contract information from manufacturers is summarized. This information was used to support one utility's blower technology decision and the hope is this information will be useful to other utilities as well.

SpeakerKraemer, Jeremy

Presentation time

09:30:00

10:00:00

Session time

08:30:00

10:00:00

SessionThe Big Bad Blower: Huffing and Puffing Air Through Your Aeration Basins

Session locationRoom S401d - Level 4

TopicEnergy Production, Conservation, and Management, Facility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design

Author(s)

Kraemer, Jeremy

Author(s)J. Kraemer 1; J. Kraemer 1 ; T. Jenkins 2; T. Young 3; C. Pretorius 4;

Author affiliation(s)GHD 1; GHD 1 ; JenTech Inc 2; GHD 3; GHD 4;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159070

Volume / Issue

Content sourceWEFTEC

Word count11

When Could You Possibly Operate An Anaerobic Digester Without Mixing?

Abstract

BACKGROUND
It has been shown that after secondary treatment, anaerobic digestion is tied for the second highest consumption of electrical power at an average water resource recovery facility (WRRF; SAIC, 2006). Despite that, there has been limited research on how to optimize digester mixing. Many design engineers and regulators take the approach that it is better to err on the side of excess mixing power. The implicit assumption is that excessive mixing has little or no detrimental effect. That may not be a good assumption. In a landmark study on digester foaming, Miot et al (2013) showed that reducing the fraction of time the digester mixing system was in operation in an egg-shaped digester reduced foaming and variability in the daily biogas flow without reducing the total biogas flow. Eventually the best results were achieved with the mixing system turned completely off. There is also much anecdotal evidence that at least some digesters function well without external mixing. Occasionally, digester mixing systems break down and the error is only discovered much later often without impacting digester performance. In other cases, owners and operators may not have the budget available to fix a mixing system that fails unexpectedly or prematurely and are forced to operate the digester without mixing, sometimes for years or even decades, without obvious signs of loss of performance. Collectively these cases raise the question of whether anaerobic digesters can be operated without external mixing, relying only on the natural mixing provided by the biogas generated in the digester (and mixing from the heat loop) or if it is only possible under certain conditions.
METHOD
Digestate in a WRRF is a non-Newtonian, shear-thinning fluid, in that its apparent viscosity reduces at higher shear rates (Taylor et al., 2020). As the shear rate is increased from near zero, there is initially a sharp drop in apparent viscosity, but eventually there is little change with increasing shear rate. One theory for optimizing digester mixing is to operate the digester at a point where there is a limited response to changes in shear rate without approaching the point of diminishing returns (Krugel & Stallman, 2018). Presumably operating under these conditions would limit the variability of viscosity throughout the digester. The authors suggested two distinct rates of response (slopes in the curve) that might be targeted for successful operation: -0.33 and -0.50 Pa.s2, as shown in Figure 1. The general applicability of these targets remains to be confirmed experimentally. Rheological testing of a sludge sample at different solids concentrations allows a series of curves, such as shown in Figure 2, to be developed. This means that the shear rate and the corresponding minimum mixing intensity can be determined at each different concentration. Figure 3 shows the minimum mixing intensity for three different plants as a function of digester solids concentration. Note that the rheological testing at Plant A used two different cup and bob sizes in the rotational viscometer, and those produced slightly different results, confirming the limitations of current test protocols. Both Plants A and B digest primary sludge only, but their measured viscosities are quite different: at around 4% total solids, the estimated minimum mixing power for both plants are similar, but it is obvious that at higher concentrations the requirement for Plant A increases much faster than for Plant B. Plant C, digesting only waste activated sludge (WAS), has much higher sludge viscosity and requires much more mixing intensity. This confirms that rheological properties vary significantly from between WRRFs, so that it is necessary to test the rheology at each WRRF. Pre-treatment, such as thermal hydrolysis, may be used to reduce sludge viscosity. Pretorius et al. (2018) showed how to calculate the mixing power provided by the biogas flow in digesters. The procedure can be reversed, to estimate the minimum gas flow required to provide the mixing intensity calculated from sludge rheology. The gas flow can be used to estimate the minimum volatile solids loading rate required, based on an assumed volatile solids reduction (VSR) percentage and a gas yield (m3/kg VSR or cf/lb VSR). This is not a new approach: the USEPA (1976) stated that if a volatile solids loading rate of 6.4 kg/m3.d (0.4 lb/cf.d) could be maintained, natural mixing would be sufficient. Depending on feed sludge concentration and volatile fraction, that loading rate would translate into an HRT of approximately 6.5 days, confirming that this is an aggressive loading rate. The VSR depends on the hydraulic retention time (HRT) in a once-through digester. The HRT, in turn, is directly related to the volatile solids loading rate. An alternative way to present the conditions required for natural mixing to suffice is as a maximum HRT.
RESULTS
Of the three plants evaluated, only the Plant A data yielded viable results for relying on natural mixing. The Plant B data, determined for a recuperative thickening project, started at a relatively high solids concentration of 3.8% where a viable solution would require a specific digester design, such as a deep digester or a high aspect ratio. For Plant C the viscosity throughout the tested range (2.9% and above) was too high to yield a viable solution. Figure 4 shows the minimum volatile solids loading rate required for natural mixing as a function of digester solids concentration, based on the rheological data presented in Figure 2. The figure suggests that up to a solids concentration of approximately 2.5%, natural mixing should suffice without excessive volatile solids loading rates. The figure also shows that for this particular sludge, the criterion recommended by the USEPA (1976) of 6.4 kg/m3.d (0.4 lb/cf.d) is quite conservative and would allow natural mixing to suffice up to a solids concentration of almost 4%. Figure 5 shows that maximum allowable HRT for the same rheological properties. The figure suggest that a target HRT of 15 days can be maintained up to a concentration of approximately 3%. The full manuscript will include a discussion of the impact of SWD and digester shape on the mixing requirements. Other factors, including grit accumulation and blending of the feed into the digester contents will also be addressed.
CONCLUSION
A method is presented that allows a comparison to be made between the mixing provided by biogas generation in the digester and the mixing required to ensure a relatively even digester viscosity to be maintained. The results show that site-specific sludge viscosity determines whether natural mixing by itself could be sufficient. The method would allow significant power savings at WRRFs to be realized. Eliminating or significantly reducing the digester mixing power would allow WRRFs to make significant progress towards achieving energy neutrality or positivity.

A method is presented to compare the natural mixing intensity to an estimated mixing requirement based on sludge rheology at different solids concentrations. For one particular WRRF the results suggest that natural mixing should suffice as long as the solids concentration in the digester remains below 2.5%. However, sludge viscosity varies significantly from WRRF to WRRF, so site-specific testing of rheology is needed to optimize digester mixing. Controlling grit and foaming is also discussed.

SpeakerPretorius, Coenraad

Presentation time

14:25:00

14:40:00

Session time

13:30:00

15:00:00

TopicIntermediate Level, Biosolids and Residuals, Energy Production, Conservation, and Management, Research and Innovation

Author(s)

Pretorius, Coenraad

Author(s)Coenraad Pretorius1; Carlo Modulon2; David Solley3; Duncan Taylor4; Jay Surti5

Author affiliation(s)GHD, Irvine, CA, USA1; GHD, Melbourne, VIC, Australia2; GHD, Brisbane, QLD, Australia3; GHD, Sydney, NSW, Australia4; GHD, North Wales, PA, USA5

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158660

Volume / Issue

Content sourceWEFTEC

Word count11