Solids are wasted from the primary clarifiers directly to the digesters at 2.16% TS. Waste activated sludge (WAS) is thickened on one of two gravity belt thickeners (GBTs) and fed to the anaerobic digesters at 5.86% TS. Sludge wasting values and concentrations for January through December 2018 are summarized below in Table 4-10.
Table 4-10: Sludge Wasting Operating Parameters (Jan – Dec 2018)
Parameter Units Primary Sludge Thickened WAS Combined Digester Feed
Flow GPD 44,400 41,500 85,900
Loading PPD 7,990 20,300 28,200
% TS % 2.16 5.86 3.94
% VS/TS % 87.2 84.4 85.2
The digesters are loaded based on a time-controlled valve sequence where both primary sludge and thickened WAS are delivered simultaneously. Primary sludge is introduced by blending to a mixing line and thickened WAS is delivered separately through an upper feed line into each digester. The three anaerobic digesters have different volumes, so each digester is fed for a different duration each day to allow for consistent hydraulic and volatile solids loading rates.
4.8.1.1 Current Digester Performance
The HCTP reports an average volatile solids reduction (VSR) of 60% during the period of June through October 2018, as shown in Table 4-11. FOG was not fed during this period, during which time the total and volatile solids loading to the digesters dropped off from what is normal, but VSR remained consistent.
Section 4-36
Table 4-11: Digester Performance without FOG (June – Oct 2018)
Parameter Units Digester In Digester Out Reduction
TS PPD 26,300 12,000 14,300
VS PPD 22,300 9,000 13,300
% TS % 3.99 1.82 54.3
% VS/TS % 84.8 75.0 59.7
The next step in determining whether the VSR rate is accurate is to compare historical gas production with historical VSR and also generate a comparative mass balance of solids into the HCTP versus the cake leaving the dewatering system.
The operating temperature of an anaerobic digester impacts the rate of volatile solids reduction, and hence gas production, as well as pathogen deactivation. For optimum mesophilic digestion, digester temperature should be maintained in the range of 95 to 98oF. At 95oF the volume of gas produced is optimal at about 20 cubic feet per pound of volatile solids destroyed as noted in Figure 4-18, (although most plants do not realize greater than 17 cubic feet per pound VS). If the temperature drops to 85oF, the gas production will correspondingly drop to about 14 cubic feet per pound of volatile solids reduced. As shown in Figure 4-18, as digester temperature increases towards the thermophilic range, a similar drop in digester gas production will occur. Based on the data received for 2018, it appears that the gas production per pound of VS reduced at HCTP averages 18 cubic feet, which is above average for the industry.
Figure 4-18: Effect of Temperature on Gas Production
Source: Adapted from Effects of Temperature on Methanogenesis in a Thermophilic Anaerobic Digestor, Zinder et al, 1984
Per discussions with plant staff and review of historical data, the temperature of the existing digesters has remained extremely consistent, varying between 96oF and 98oF as shown in Figure 4-19 below. Digester 3 was taken out of service for rehabilitation in July 2018, which explains the lack of data shown between July and December 2018. According to Figure 4-18 above, showing
Section 4-37
the peak gas production rate relative to digester temperature, by maintaining a consistent mesophilic temperature for Digesters 1 and 2 during 2018, the City was able to maintain gas production at peak production levels. It is noted that a number of temperature measurements in January, March, and October of 2018 fall below the optimal minimum temperature of 95oF. The explanation provided by the City for this diversion is that a reduction in FOG loading resulted in reduced digester gas production, meaning that the IC engine did not generate adequate waste heat and the digesters had to be supplemented with heat from the natural gas-fired boiler.
Figure 4-19: Digester Temperature (Jan – Dec 2018)
Volatile solids reduction can vary from plant to plant depending on the consistency of digester heating and the method of digester mixing employed. Plants that utilize pumped mixing systems, such as HCTP, typically experience VSR rates in the range of 55% to 65%. It appears that the City safely falls in this range and the digesters are being heated and mixed at an optimal level.
4.8.1.2 Enhancing Digester Performance
Digester gas generation is limited to conversion of the readily degradable portion of wastewater solids. In order to make solids more readily degradable, there are a variety of digester pretreatment technologies. Digestion pretreatment technologies improve the digestibility by modifying the microbial cells through perforating or lysing the cell walls. This increases the volatile solids reduction achieved in anaerobic digestion and increases the gas production. Because, pretreatment typically results in minimal improvement in digestion of primary solids, many of these technologies are applied only to secondary sludges such as waste activated sludge (WAS).
Pretreatment technologies include thermal hydrolysis, electrical pulse treatment, sonication, and mechanical disintegration. Pretreatment technologies have shown to almost double the readily biodegradable fraction of the volatile solids in certain secondary sludges, resulting in a 20% to 40% increase in gas production compared to standard anaerobic digestion.
• Thermal Hydrolysis Process (THP)
In thermal hydrolysis, steam is injected into wastewater solids at high pressure to rupture cells and enhance the conversion of organic matter to digester gas. The application of THP is the most
90
1/1/2018 1/31/2018 3/2/2018 4/2/2018 5/2/2018 6/2/2018 7/2/2018 8/2/2018 9/1/2018 10/1/2018 11/1/2018 12/1/2018 1/1/2019
Temperature (oF)
Digester 1 Digester 2
Section 4-38
widely adopted and most mature technology, having been implemented for over 25 years. There are currently over 30 installations of the Cambi® THP system, and close to 10 installations of the Veolia Exelys process in operation or under construction in Europe. These are the two main manufacturers of THP systems.
Thermal disintegration processes have higher energy consumption (i.e., electricity, digester gas, and natural gas) than mechanical methods, but they can potentially use waste thermal energy instead of other energy sources. The input of thermal energy is generally achieved using heat exchangers or by application of steam to the sludge. Excess heat obtained within the plant can be used, thus reducing the energy cost of pretreatment significantly.
THP has been studied under temperatures ranging from 140 to 520°F. Temperatures above 400°F have been found to result in the formation of refractory compounds and are generally avoided. The most common treatment temperatures reported in the literature are between 140 to 360°F. Treatments applied at temperatures <212°F are considered low temperature thermal treatments. There is evidence that thermal treatment in the high thermophilic range (>160°F) may enhance the activity of the bacterial population, improving the degree of digestion. This, however, is not generally considered THP because the cell walls are not disrupted by the action of heat, rather they are acted upon more effectively by biological enzymatic hydrolysis. A schematic of THP is shown in Figure 4-20.
Figure 4-20: Thermal Hydrolysis Schematic
Source: Veolia Exelys
Primary and secondary sludges are dewatered to concentrations as high as 20% TS prior to entering the THP reactor, where they are heated with steam at pressures above 100 pounds per square inch (psi). The sludge then transitions into an expansion tank where a sudden drop in pressure helps rupture the cell walls of the organic material. Due to the hydrolysis that occurs, the solids concentration drops in half (down to 10% TS) prior to the digesters. Heat exchangers are utilized to cool the sludge and prevent the digester from reaching thermophilic temperatures, which can result in higher odors in the dewatered cake.
Section 4-39
Pretreatment using THP generally results in digesters achieving volatile solids reduction of 60%
or greater, and increased gas production by 20% to 30%. It has been documented that the THP conditioning process significantly improves the dewaterability of the biosolids after digestion, producing a drier cake, which in turn results in a substantial reduction in biosolids hauling and disposal costs. Based on the temperatures that are reached, dewatered cake generated from anaerobic digestion with THP also qualifies as Class A biosolids.
The main drawbacks of THP are its high capital cost and operational complexity, mainly due to the high-pressure and temperature reactors. For example, a licensed steam operator is required to be on staff at the treatment plant for maintaining the high-temperature system. THP systems are also typically used at treatment plants exceeding 15 MGD due to the capital and O&M costs involved. Based off solids throughput, Cambi representatives have estimated that the THP system appropriate for HCTP is currently in operation at plants in Medina, Ohio and Pontiac, Michigan.
The digester VSR rates are currently nearing 60% so, the benefit that would come with the addition of THP would be minimal. The process is fairly complex, including pre-digestion dewatering, high temperature/steam boilers, and a series of high-pressure vessels. Based on capital cost, O&M complexity, and lack of gas production benefit, thermal hydrolysis is not recommended for HCTP.
4.8.1.3 Other Solids Reduction Alternatives
Other than thermal hydrolysis, most digester pretreatment technologies are relatively simple and have small footprints, making them relatively easy to retrofit into an existing facility. A summary of other solids reduction technologies is presented below.
• Sonication
Ultrasound is sound above the range of human hearing, with frequencies between 20 kHz and 10 MHz. At these frequencies, sound waves produce microbubbles, which then collapse (a phenomenon known as cavitation), causing high mechanical shear forces that can disintegrate bacterial cells.
There are several full-scale installation of the technology in Europe, but none in the U.S. to date. Sonication tests conducted in the U.S. by one manufacturer, Sonotronic, have shown inconsistent results. At the Orange County Sanitation District, sonication increased biogas production by 50%, while at the Joint Water Pollution
Control Plant in Los Angeles County, tests showed only a 1% increase in VSR and 7.9% increase in biogas production. An example of sonication injection equipment is shown in Figure 4-21.
• Electrical Pulse Treatment
Electrical pulse treatment is a physical pretreatment technology that uses pulsed electric field technology to disrupt cell walls. The applied electric field disrupts the lipid layer and proteins in the cell membranes, making the cell wall porous, eventually causing rupturing and release of
Figure 4-21: Sonication Equipment
(Source: Sonotronic)
Section 4-40
intercellular material for better digestion. Vogelsang manufactures this system and it is called BioCrack. Currently, there are over 10 installations in Europe but none in the US.
• Mechanical Disintegration Sludge disintegration systems, such as those manufactured by Crown, are a mechanical cell lysing system consisting of a high-speed mixer, a homogenizer, progressing cavity pumps, a recirculation tank, and a disintegration nozzle. Pressurized solids are forced through a disintegration nozzle, resulting in a sudden pressure drop that causes cavitation. The shear forces resulting from the implosion of the microbubbles cause the cell walls to rupture.
Pretreatment through mechanical disintegration appears to improve solids reduction and biogas production during
anaerobic digestion, as well as reduce foaming potential by disrupting filamentous bacteria. There are over 20 Crown disintegration system installations, mostly in Europe, Australia, and New Zealand. The only U.S. installation was installed in 2015 in the City of Visalia, CA. Current data shows that the gas production has increased by 15 to 20% with operation of the Crown system.
An example of mechanical disintegration equipment is shown in Figure 4-22.
Comparing THP against the other three pretreatment technologies, as shown in Table 4-12, THP is the only process that generates a Class A product. However, it is also the highest capital cost and includes the most O&M complexity. Budgetary pricing information for installing a system at the HCTP was requested from the manufacturers listed above, in addition for projections on additional VS reduction that can be expected based on the data analyzed for 2018. Manufacturers may request sludge samples in order to provide estimated system sizing and pricing information.
The results of that request are included in the Table 4-12 below.
Table 4-12: Comparison of Sludge Pretreatment Technologies Variable Thermal
Hydrolysis Electrical Pulse
Treatment Sonication Mechanical Disintegration Maturity Established Innovative Established Innovative
VSR Increase 20-30% 10-20% <10% <10%
Currently, digester VSR rates nearing 60% and the benefit that would come with the addition of other sludge pretreatment technologies is minimal. The most cost-effective means to boost gas Figure 4-22: Mechanical Disintegration Equipment
(Source: Crown Biogest)
Section 4-41
production appears to be continuing the addition of high-strength wastes, such as FOG and FW.
Modifications to the digestion process, including cell lysing, should only be considered in the future if gas production rates drop significantly or if the digested sludge becomes difficult to dewater. Budgetary pricing for a THP system was received from CAMBI. The THP equipment cost alone is $2.6 M, and with additional support infrastructure, installation, electrical and instrumentation, and markups associated with design and administration the estimated cost for the HCTP would be around $9.8 M. This project is included in the proposed ten year CIP as Project #31 in FY 2027/28 and FY 2028/29 budget, in case the HCTP needs to implement this system in the future.
Currently, the digesters are performing very well, and the City has rehabilitation projects planned in the latest CIP for all three digesters to keep their performance at a high level. When these facts are considered, the HCTP does not need to invest in a sludge pretreatment technology. However, if in the future another digester is added into the system or the performance of digesters start going down, then a pretreatment technology could be considered.
Additionally, there are some operational modifications that could be implemented at the HCTP to improve the digestion system performance at a lower cost than a pretreatment technology as discussed in Section 4.8.2.2.
Dewatering and Permeate Treatment
Currently the HCTP periodically pumps anaerobically digested sludge to the Dewatering Building, where it is blended with polymer, and sent to screw presses for dewatering. Although the screw press manufacturer’s data suggest 19% for sludge cake solids concentration is achievable, actual values are somewhat less at around 16% solids concentration. The screw press was originally designed for a hydraulic loading rate of 87 gpm and 867 lbs/hour.
The digested solids are conveyed to the dewatering system over roughly 13 to 16-hour period per day cycle. Ideally, the screw presses can be operated 24 hours per day, however, the dewatering operations are suspended overnight. During dewatering operations, digested solids are pulled from digesters sequentially. Digested solids are pulled from different digesters during a typical dewatering cycle and vary in solids concentrations by up to 15% according to start-up logs of the dewatering equipment technical services representatives. This amount of variation in solids content is unusual. It may suggest that either each digester shows high variation in solids concentration or one of the digesters is underperforming. It is suggested that the solids content of each digester to be observed and if a digester is underperforming, the mixing system should be investigated to ensure it is performing properly. Underperformance could also be the result of built-up grit, plastics, or other solids in the digester, which can be corrected through cleaning.
Digester 3 is currently undergoing cleaning and rehabilitation. Digesters 1 and 2 are scheduled to follow over the next 2 years. Performance should be reevaluated following completion of this work to confirm improvement.
During the dry season, the reduced cake solids content produced by the screw press is essentially negated through supplemental solar drying. However, during the rainy season, it is recommended the HCTP to optimize their dewatering operations with respect to solids concentrating to reduce truck hauling cost. Optimizing performance of the dewatering process to improve capture would be beneficial year-round. A testing plan is recommended to quantify the range of digested sludge solids concentrations particularly when switching from one digester to the next.
Dewatered sludge cake solids content and capture varies depending on digested sludge solids concentration, polymer to sludge ratio, and other factors. Although the polymer dosing is operator adjustable, it does not appear that the infrastructure is in place to automate this process. Actively adjusting the polymer dosage can improve the cake solids content and process capture rate.
Section 4-42
Permeate produced from the dewatering process is sent directly to a small open top tank in the dewatering room before being returned to the designated side stream bioreactor, one of Bioreactors 3 through 6. Operators have indicated that they have experienced frequent pipeline plugging and pump clogging in the return line from the dewatering building in the past. The plugging was caused by the formation of a crystalline structure of magnesium ammonium phosphate hexahydrate, more commonly known as struvite (discussed further in Section 4.8.2.3).
To maintain operations, operators were required to regularly clean the permeate transfer pumps and pipeline. The solids dewatering process was retrofitted with this open tank to provide permeate dilution to inhibit struvite formation.
Permeate is currently pumped directly to the designated side stream bioreactor to reduce the ammonia prior to being discharged to the primary effluent chamber. Because of the addition of the dilution tank, struvite formation has subsided. However, the tank has brought about other challenges, which include frequent manual removal, collection, and disposal of floating solids.
Plant operators has noted that there is uncharacteristically high level of solids in the permeate.
This could be attributed to several factors such as insufficient polymer aging, too high of a loading rate, or some other mechanical issue that is allowing solids to not be captured in the screens of the screw press. Figure 4-23 shows the permeate dilution tank in operation and the high level of solids which collect along the tank surface.
Figure 4-23: Permeate Dilution Tank and Floating Solids
Section 4-43 4.8.2.1 Process Optimization
Although dewatered cake solids content is less than expected, on-site supplemental solar drying essentially negates the majority of negative impacts, except during the rainy season, when excess water must be hauled and disposed of. Potentially dewatered cake solids content and solids capture could be improved through various types of process optimization techniques. Process optimization modifications that may alone or in combination increase dewatered cake solids and/or improve solids capture rate are as follows:
• Reduce the speed of the screw press
• Optimize polymer feed rate
• Test alternative polymer types
• Improve sludge/polymer blending
• Modify polymer feed point
• Increase digested sludge solids content
• Operate screw press for extended periods of time
Observe cake solids and capture with process optimization and if needed proceed with process improvements.
4.8.2.2 Process Improvements
If process optimization techniques described above are not practical or do not adequately improve processes performance, improvements to the current dewatering system may be needed.
Typically process optimization improvements are preferred because they can be made with little to no capital investment, while the improvements discussed below may represent more significant CIP investment.
• Monitoring
The screw press feed sequence cycles from each of the three digesters one at a time, which may cause the feed solids content to rapidly change. Solids content fluctuation of the feed may contribute to reduced cake solids content and poor solids capture rate. Installation of modern instrumentation to monitor the solids content in the feed should be considered to provide continuous real time solids content feed data. Solids probes like the YSI VI Solids 700 IQ utilize an optical measurement window made of sapphire to provide accurate total suspended solids measurement in real time. Solids probes could either be installed on each of the digester withdrawal pipelines or at a single point along the screw press feed line. The probes are fairly
The screw press feed sequence cycles from each of the three digesters one at a time, which may cause the feed solids content to rapidly change. Solids content fluctuation of the feed may contribute to reduced cake solids content and poor solids capture rate. Installation of modern instrumentation to monitor the solids content in the feed should be considered to provide continuous real time solids content feed data. Solids probes like the YSI VI Solids 700 IQ utilize an optical measurement window made of sapphire to provide accurate total suspended solids measurement in real time. Solids probes could either be installed on each of the digester withdrawal pipelines or at a single point along the screw press feed line. The probes are fairly