Biosolids Processing Report City of Buffalo, Minnesota

Transcription

1 April 2017 Biosolids Processing Report City of Buffalo, Minnesota W Submitted by: Bolton & Menk, Inc Highway 12 East Willmar, MN P: F:

2 Certification Biosolids Processing Report for City of Buffalo, Minnesota W April 2017 I hereby certify that this plan, specification or report was prepared by me or under my direct supervision, and that I am a duly Licensed Professional Engineer under the laws of the State of Minnesota. By: Bradley C. DeWolf, P.E. License No Date: April 19, 2017 Biosolids Processing Report City of Buffalo, MN ǀ W Certification

3 Table of Contents I. INTRODUCTION... 1 II. PERMIT COMPLIANCE... 2 III. BIOSOLIDS STORAGE TANKS... 3 IV. BELT PRESS DEWATERING SYSTEM... 5 V. BIOSOLIDS DRYER EQUIPMENT... 7 VI. CONVEYORS AND DRY SOLIDS SILO... 8 VII. INCINERATOR AND ASH CONTAINER... 9 VIII. DRYING SYSTEM DUCTWORK IX. UTILITY WATER FILTERS X. ACTIVATED CARBON AND LIME FEEDERS XI. COST ESTIMATES XII. SCHEDULE Figures Figure 1 Biosolids Tanks Locations... 4 Figure 2 Biosolids Building Plan... 6 Figure 3 Utility Water Screen Tables Table 1 - Cost Estimate Table 2 20-Year Life Cycle Analysis Biosolids Processing Report City of Buffalo, MN ǀ W Table of Contents i

4 I. INTRODUCTION The City of Buffalo last reviewed its biosolids processing in the early 2000 s. After a few years of research the Wastewater Treatment Facility Evaluation was completed in August of After this, design on the biosolids processing improvements began. The City of Buffalo constructed the drying facility upgrade in The dryer began operation in the fall of 2008 and has been in operation since then. The City has also began processing biosolids from other communities in recent years. The biosolids storage tanks and mixing system vary in age but are a minimum of 25-years old. This report will review the process equipment and infrastructure of the biosolids facilities new technologies available and provide recommended process improvements and estimated costs for the related upgrade. INTRODUCTION Biosolids Processing Report City of Buffalo, MN ǀ W Page 1

5 II. PERMIT COMPLIANCE The City of Buffalo currently holds two permits regarding discharges from the Wastewater Treatment Facility. One is the National Pollutant Discharge Elimination (NPDES) Permit No. MN regulating water discharges to the receiving water. The other is Air Emission Permit No This permit regulates emissions from the biosolids incinerator and backup electrical generators. The facility has an exceptional record of compliance with these limits for both the NPDES and Air Emission permits. The biosolids treatment process is critical to continued compliance with these permits. The improvements described below all affect the process efficiency of the biosolids treatment process and the related quality of discharges and emissions. The improvements in particularly affect the air emissions quality, treatment process and permit compliance. Completion of these improvements will enhance the facilities performance and ability to comply with air emissions limits. PERMIT COMPLIANCE Biosolids Processing Report City of Buffalo, MN ǀ W Page 2

6 III. BIOSOLIDS STORAGE TANKS The City has multiple biosolids storage tanks. Locations of these are on the site plan included as Figure 1. Many of these are older treatment process tanks converted to biosolids storage tanks with coarse bubble aeration for mixing. These concrete tanks are generally in good shape and require no significant maintenance. The City has a glass-lined steel tank, referred to as Big Blue, constructed in 1991 that is used as the final storage and mixing tank prior to the drying process. This tank has a jet nozzle pump mix system over 25-years old, originally designed to be used on a seasonal basis. The nozzles are designed to be adjustable but are difficult to operate due to age. Staff has found the drying process works best with this tank used as the primary storage tank since the jet mix system has less effect on the volatile content of the biosolids than the aeration mixing system. Higher volatiles result in more heat from the incinerator and less natural gas usage. This tank also receives biosolids from other communities that is often quite thick and more difficult to mix. A variety of mixing systems are available for this type of application. The selected option is a newer style jet mix system which best utilizes existing piping and pump room while providing a complete mix. The new pumps would require a variable frequency drive (VFD) to allow adjusting the mixing rate to the tank level for more efficient operation. The electrical service will likely require an upgrade to accommodate the larger horsepower pumps. The tank is equipped with a cathodic corrosion protection system that utilizes sacrificial zinc anodes. The anodes require periodic replacement to keep this protection system operating. The anodes are currently ready for replacement and should be replaced while the tank is off-line for the mixing system upgrade. The other transfer pumps and piping associated with these tanks are in good condition with no improvements required. BIOSOLIDS STORAGE TANKS Biosolids Processing Report City of Buffalo, MN ǀ W Page 3

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8 IV. BELT PRESS DEWATERING SYSTEM The biosolids are pumped from Big Blue to a small wetwell which is connected to the belt press feed pumps. The belt presses dewater the biosolids to approximately a 20% cake. This cake is conveyed via shaftless screw augers to the wet cake bunker. This system is generally operating very well. The wetwell does see some settling issues with biosolids, particularly with heavy solids from other communities. It is proposed to add a small submersible wetwell mixer to keep solids in suspension. This mixer is similar to other submersible mixers the City uses in the scum wetwell. The location of the proposed mixer is shown on the Biosolids Processing Building Plan, Figure 2, highlighted area #1. The shaftless screw augers are in generally good condition. They do require periodic replacement of seal which has been ongoing by staff. An access platform was also recently added to provide better access for staff to replace seals. The conveyors are stainless steel construction and are in good condition. The wet cake bunker provides short-term storage of cake to be pumped to the dryer. It is in good condition. The only item proposed for upgrade is the level sensor that detects high and low levels in the bunker. The current sensor is an ultra-sonic style. It is currently limited in ability to detect very high or low levels. Technology improvements have made radar level sensors more accurate and robust for this difficult environment. Radar sensors have been successfully used on other hoppers by staff. It is proposed to replace this sensor with a radar style level sensor. This in shown on Figure 2 as item #2. BELT PRESS DEWATERING SYSTEM Biosolids Processing Report City of Buffalo, MN ǀ W Page 5

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10 V. BIOSOLIDS DRYER EQUIPMENT The dryer equipment includes the dosing pumps, drive belts and insulated cabinet. The dosing pumps have a few items that require routine maintenance but are in good working condition and require no significant repairs. The dryer has two sets of stainless steel belts. The belts are in good condition showing very little wear and require no work. The belts are driven by drive shafts with gear reducer assemblies. The gear reducers are in working condition. Pricing has been obtained to replace the belts, gear reducers and brass rails. The brass rails are a wear component and should be replaced based on hours of use and visual inspection showing wear. The belts are still in good condition and would not require replacement at this time. This is shown on Figure 2 as item #3. The dryer cabinet is a steel structure with insulation and metal cladding. The cladding is showing rust and corrosion at a few locations. It appears some condensation may be occurring and leaking behind the cladding causing the rust and corrosion. This is most evident near the dosing pumps and at the bottom of the cabinet where condensation appears to get trapped. The cladding was recently stripped off a small area at the worst location for inspection. The dryer cabinet was showing some rust and coating failure. A smoke test was also performed to check for leaks. The smoke test showed minor leakage around inspection door seals. It also showed some leakage around the depositor end where the corrosion is the worst. During the drying process, the interior dryer cabinet is filled with hot humid air. Condensation is formed wherever this humid air contacts a cooler surface. Any gas leak will create condensation within the insulation layers. This moisture eventually causes rust and corrosion. The dryer cabinet is a painted steel structure covered with 4-inches of rockwool insulation and a thin painted steel cladding to hold the insulation in place. The cladding should be fully stripped to allow inspection of the dryer cabinet coatings. The affected areas will require a sandblast and recoating. The cladding would be replaced and installed in a manner to allow free escape of humid air and condensation. This area is shown on Figure 2 as item #4. The shaft bearing seals show some smoke leakage. The bearings are in good condition but it is apparent there is a leaking seal. Staff has obtained a quote for Lovegreen Industrial Services to inspect these seals for $2,394. It would be recommended to research this prior to a bid so the magnitude of replacement seal parts can be determined. The door seals should also be replaced. The seals are rated for high temps but get stiff with age. The dryer also has a smaller flanged duct connection connected to the condenser that is showing visible corrosion. This is assumed to require replacement of the flange section. This is shown on Figure 2 as item #5. BIOSOLIDS DRYER EQUIPMENT Biosolids Processing Report City of Buffalo, MN ǀ W Page 7

11 VI. CONVEYORS AND DRY SOLIDS SILO The dryer is connected to a dry solids silo by an auger. This is a shafted steel auger that is in a steel trough. The steel trough has corroded through and been patched in areas. It appears the dried solids create corrosive air that has caused the steel trough to corrode. This is proposed to be a material upgrade to a stainless steel trough. The auger shaft and flighting are not showing corrosion but the flighting is beginning to erode from friction. Replacement of the shaft with a harder stainless steel material at the same time as the trough would be proposed. This is shown on Figure 2 as item #6. The dry solids silo is in good shape with no maintenance required. The auger from the silo to the furnace is a tube style auger that is corroding. This section would also be proposed to be upgraded with a stainless steel unit. This is shown on Figure 2 as item #7. The conveyors are maintenance items requiring urgent attention. These may be required to be installed via separate contract prior to the larger biosolids improvement project. CONVEYORS AND DRY SOLIDS SILO Biosolids Processing Report City of Buffalo, MN ǀ W Page 8

12 VII. INCINERATOR AND ASH CONTAINER The biosolids incinerator major wear component is the grates in the bottom of the furnace. These are moving grates that oscillate and move the ash towards the extraction screw. The ash is abrasive and cause wear in the grates. These are currently functioning but showing wear should be planned for replacement. Staff has obtained a quote for replacement grates through KMW Energy, Inc. from Ontario, Canada. This is shown on Figure 2 as item #8. Changing the grates is considered a low priority as they are working, but purchasing replacement grates to have on-site would be recommended. Delivery time for these is 6 weeks or longer and would result in extended down time in the event of a failure. The flue gas connection to the furnace has had a tendency to trap soot and block the ductwork. As the duct plugs, air flow rates and fan speeds need adjustment affecting process efficiency. It can also eventually require shutdown for cooling and manual cleaning. A soot blower can be added to this location to automatically blow the soot out and keep this location open and maintain free air movement. The soot blower is a relatively low cost item but it will require some duct modifications. This is a high temperature rated duct with a cured-in-place cementitious coating that will need to be repaired. This is shown on Figure 2 as item #9. The ash extracted from the dryer is augured to a roll-off dumpster and stored until it is hauled to a landfill. The dumpster is currently shrouded in a curtain to contain dust. This is marginally effective but is open at the top and allows some dust to escape. Tight fitting covers with self levelling augers have been reviewed. Staff has identified a company that can custom design a cover for the dumpsters the City uses. This would be a self levelling cover constructed of 304 stainless steel with dust filters. This is shown on Figure 2 as item #10. INCINERATOR AND ASH CONTAINER Biosolids Processing Report City of Buffalo, MN ǀ W Page 9

13 VIII. DRYING SYSTEM DUCTWORK The dryer, incinerator, heat exchanger, bag filter and exhaust stack are connected with a series of ductwork runs. The critical large ductwork runs connected to the filter are stainless steel and have been successfully smoke tested showing these sections are in good condition. The ductwork is generally in good shape with the exception of one aluminum run from the incinerator to the dryer. Condensation in this duct corrodes the aluminum and has corroded the majority of this run. It would be proposed to upgrade this section with a stainless steel material. This is shown on Figure 2 as item #11. All other duct runs appear to be in good condition. The exhaust stack cladding is showing corrosion. It would appear there is some condensation or exhaust leak on fittings to the stack. The cladding should be stripped and stack connections checked/repaired. The replacement cladding should be equipped with openings to allow condensation to escape and reduce corrosion. The ducts are connected to fans and equipment with expandable joints to absorb vibrations and allow thermal expansion. These are aging and getting brittle. These would be recommended for replacement as part of the project. The incinerator has one air flow sensor inline in the overfire air duct that is currently not functioning. This should be replaced. This is shown on Figure 2 as item #12. DRYING SYSTEM DUCTWORK Biosolids Processing Report City of Buffalo, MN ǀ W Page 10

14 IX. UTILITY WATER FILTERS The utility water system currently has a bag filter system located in the drive through garage. The bags require frequent replacement and are not self-cleaning. This can create issues on nights and weekends when the facility is not staffed. Automatic backwashing systems are available and have been reviewed previously by staff. Any system would be preferably installed in the RAS building near the utility water pumps and would filter the utility water for the whole facility. This was previously reviewed with a proposed filter design laid out in the RAS building as shown on Figure 3. The RAS building has a drain sump with one sump pump currently. In order to accommodate the increased flows from this backwash process, a larger sump with duplex pumps will be required as part of this installation. UTILITY WATER FILTERS Biosolids Processing Report City of Buffalo, MN ǀ W Page 11

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16 X. ACTIVATED CARBON AND LIME FEEDERS The powdered activated carbon and lime feeders share a common bag framework with bag hooks, lifting system and volumetric feeders. Reagent usage is monitored using the speed control of the feeders to calculate the feed rate. The feeders do occasionally plug and give false readings. The readings are critical for the bag filter operation and ultimately the air emissions permit documentation. Process performance and permit compliance could be improved by adding load cells to track the weight of the bags and reagent usage. The addition of load cells would require modifying the existing two-bag framing system into two free standing units. This will require some structural modifications along with the load cells. This is shown on Figure 2 as item #13. ACTIVATED CARBON AND LIME FEEDERS Biosolids Processing Report City of Buffalo, MN ǀ W Page 13

17 XI. COST ESTIMATES The estimated costs for the improvements discussed above are presented in Table 1 below. The costs include all items discussed. Table 1 - Cost Estimate Item Cost Mobilization, Bonds, Insurance $140,000 Biosolids tank mixing system $445,000 Wetwell Mixer $37,000 Wet Cake Bunker Level Sensor $9,000 Dryer Cabinet $139,000 Cladding and Cabinet Coatings $180,000 Conveyors and Dry Solids Silo Auger $71,000 Incinerator and Ash Container $295,000 Drying System Ductwork $90,000 Utility Water Filter and Sump $95,000 Activated Carbon and Lime Feeders $117,000 Subtotal $1,618,000 Contingencies 20% $320,000 Engineering 20% $320,000 TOTAL COST $2,258,000 The improvements will all have a minimum 20-year life cycle with exception of certain sacrificial wear items (brass rails, etc.). Table 2 presents a 20-year life cycle cost of the proposed improvements comparing the proposed improvements constructed with stainless steel components versus utilizing lower cost carbon steel and aluminum components. The stainless steel is a more expensive material but will have a significantly lower life cycle as compared to the lower cost carbon steel and aluminum components. Table 2 shows that it is more cost effective to utilize the stainless steel materials versus using lower cost materials but replacing more frequently. COST ESTIMATES Biosolids Processing Report City of Buffalo, MN ǀ W Page 14

18 Table 2 20-Year Life Cycle Analysis Proposed Improvements (Stainless Steel Materials) Item Useful Replacement Total 20-Year Cost Life Cost (1) Cost (1) Mobilization, Bonds and Insurance 140, ,000 Biosolids Tank Mixing System , ,000 Wetwell Mixer 20 37, ,000 Wet Cake Bunker Level Sensor 20 9, ,000 Dryer Cabinet Brass Rails , , ,000 Dryer Claddings and Coatings , ,000 Conveyors 20 71, ,000 Incinerator Grates 10 80,000 80, ,000 Ash Container and Blower , ,000 Drying System Ductwork 20 90, ,000 Utility Water Filter and Sump 20 95, ,000 Activated Carbon and Lime Feeders , ,000 Contingencies , ,000 Engineering , ,000 Total 2,258, ,000 2,477,000 (1) Replacement and Total Costs based on 2017 costs. No inflation adjustment. Improvements with Carbon Steel and Aluminum Item Useful Replacement Total 20-Year Cost Life Cost (1) Cost (1) Mobilization, Bonds and Insurance 140, ,000 Biosolids Tank Mixing System , ,000 Wetwell Mixer 20 37, ,000 Wet Cake Bunker Level Sensor 20 9, ,000 Dryer Cabinet Brass Rails , , ,000 Dryer Claddings and Coatings , ,000 Conveyors 10 55,000 55, ,000 Incinerator Grates 10 80,000 80, ,000 Ash Container and Blower , , ,000 Drying System Ductwork 10 70,000 70, ,000 Utility Water Filter and Sump 10 75,000 75, ,000 Activated Carbon and Lime Feeders , ,000 Contingencies , ,000 Engineering , ,000 Total 2,147, ,000 2,726,000 (1) Replacement and Total Costs based on 2017 costs. No inflation adjustment. COST ESTIMATES Biosolids Processing Report City of Buffalo, MN ǀ W Page 15

19 XII. SCHEDULE The proposed work is primarily equipment replacement with newer technologies. It would not change operations or treatment capacity and therefore would not require MPCA review. The work items can all be completed relatively quickly once equipment arrives. The equipment deliveries could be weeks for specialized equipment. Installation timing would be critical and all work would be required to be staged to fall between dryer runs. A typical schedule is presented below: Item Time Develop Bid Package May-June 2017 Advertise and Bid July 2017 Begin Construction August 2017 Complete Construction June 2018 SCHEDULE Biosolids Processing Report City of Buffalo, MN ǀ W Page 16