Jim and David,

Dennis has brought up this subject a number of times over the past months during my discussions with him.  The City and K/J have also discussed this topic collectively during our design review meetings.  Based on our earlier analyses, the decision was made not to include thickening.  However, Dennis is being very persistent on this topic and throwing you some new or different information, so let’s take a step back and review this decision again.  K/J has no specific preference for or against thickening.  Our only goal is to put forth the best solution for your project.  Thickening was originally included as part of the project in the Engineering Report and taken out during preliminary design after examining the economics of solids handling with and without thickening.  We offer the following analysis as a means of re-evaluating and confirming the earlier decision to exclude thickening, so that we can all confidently move ahead with the design.  As the design has progressed significantly since the last time this issue was evaluated, we were able to examine it in more detail.  For the bottom line, please refer to the financial evaluation and conclusion at the end of this e-mail.

Benefits of Thickening:

The following is a list of benefits related to sludge thickening, which are consistent with the benefits Dennis references, and most of which have cost impacts:

1)       Thicker sludge can improve sludge dewatering, which would yield more concentrated cake and reduce hauling.

2)       Thicker sludge often results in less polymer usage.

3)       Thicker sludge reduces the digestion volume required or increases the SRT (in comparison to the same volume for non-thickened sludge).

4)       Thicker sludge reduces the hydraulic loading rate on dewatering equipment, which can reduce the size/number of equipment needed.

Disadvantages of Thickening:

The following is a list of disadvantages related to thickening, specifically membrane thickening that is being suggested by Dennis, and are primarily cost related:

1)       Higher operating costs related to labor, blower energy use, and pumping energy use.

2)       Higher maintenance cost related to membrane replacement and maintaining additional equipment.

3)       Higher capital cost related to the purchase of thickening equipment, electrical wiring, instrumentation, space for housing the equipment, installation of equipment, additional piping, and additional programming.

Assumptions for Analysis:

Since all of the disadvantages are cost related and essentially all of the benefits have cost impacts, the decision really boils down to one of economics.  To perform a fair economic analysis, we will make the following assumptions:

• Process modeling predicts that un-thickened sludge will be around 1.0 to 1.2% solids.  Assume 1.1% solids.
• Based on operation of the full-scale pilot, thickened sludge averaged about 2.2% solids.  Although brief testing suggested the thickener could operate at higher levels, limitations unrelated to the full-scale pilot prevented confirmation that elevated solids concentrations could be maintained long-term.  Therefore, this analysis will be based on thickened sludge at 2.2% solids.
• Even at 2.2% solids, there will still be less than 40 days SRT, whether with current larger digesters planned or smaller versions, so lime use will still be employed.
• When the full-scale Fournier press was piloted at the WWTP, it achieved 12 to 13% cake solids with feed solids at about 1.9%.  When the full-scale Prime Solution press was piloted, it achieved around 14% solids (as measured primarily by the vendor) with about 2% feed solids on the first day of piloting and around 13% solids with about 1% feed solids on the second day of piloting.  This data suggests that there is likely a small increase in cake solids that could be expected with thickened sludge, which is consistent with reports from City staff that there was some increase in performance of the belt filter press after startup of the membrane thickener.  Given that the sludge quality will change significantly with the new WWTP, this data does not translate directly.  However, we will assume that without thickening cake solids will be 15% and with thickening would be 17%.
• In the economic analysis performed by the City for land application, the average haul cost was assumed to be $80/wet ton.  That same cost will be assumed for this analysis.  Currently, the plant produces around 3,000 lbs/day of sludge.  At the Phase 1, about 5,400 lbs/day will be produced.  The average during that period is assumed to be 4,200 lbs/day.  At 15% solids this will yield about 14 wet tons per day, for a total haul cost of about $280,000 per year at $80/wet ton, assuming 31 wet tons per week are composted.  At 17% solids this will yield about 12.4 wet tons per day, for a total haul cost of $230,000 per year at $80/wet ton, assuming 31 wet tons per week are composted.
• During pilot testing of the Prime Solution pilot unit, polymer dose ranged from 0.2 to 0.3 gph.  It will be assumed that the average dose was 0.25 gph.  Based on the feed rate of 7.5 gpm, this translates to polymer doses of 12 lbs/dry ton for 1.1% feed solids and 6 lbs/dry ton for 2.2% feed solids.  Based on Phase 1 loading of 5,400 lbs/day from the digesters, the polymer cost would be about $20,000 per year for 1.1% feed solids and $10,000 per year for 2.2% feed solids, based on the City reported polymer cost of $1.65 per lb.
• Although thicker sludge reduces the volume required for digestion, there is little savings to be realized here.  Currently, one of the existing SBR tanks is to be divided into two digester tanks.  Although one digester tank should be sufficient in size with the thicker sludge, there would be no space for sludge storage should the one digester tank be taken out of service for maintenance.  Therefore, it is recommended that two digester tanks be constructed as proposed.  However, the digester tanks could be constructed to utilize a smaller portion of the existing SBR tank.  This would reduce the size of the blowers, the number of the diffusers, and the size of the covers.  Estimated cost reduction for smaller blowers is about $50k.  The estimated cost reduction for fewer diffusers is about $55k.  Estimated cost reduction for covers is about $190k.  Total cost reduction for smaller digester is $295k.  However, there would be about $205k in additional concrete work to subdivide a smaller portion of the existing SBR tank into two digesters.
• Based on the assumed feed solids rate, the thicker sludge would reduce the hydraulic loading rate on the rotary fan presses by about half.  Therefore, rather than two units, only one unit would be needed.  Currently, the price of two units from Prime Solution unit is about $288k and the price of two similar capacity units from Fournier unit is about $545k.  The cost savings will be based on the less expensive of these two manufacturers, which is estimated to be about $155k with installation.  Adding 15% for electrical/instrumentation, the cost is estimated to be $180k for one fan press (without markups).  Also, running one versus two dewatering units would save about $2,000 per year in power costs.  It is assumed that there would be no significant difference in operations labor.
• Based on the original proposal from Enviroquip, the membrane thickener has a stated capacity of 80,000 gpd at 0.5% solids, or a loading rate of about 3,300 lbs/day.  For Phase 1, the anticipated solids loading is expected to reach about 6,600 lbs/day, which is about twice the rated capacity.  The proposal states that the tankage and blowers of the existing full-scale pilot are sized for 50% more capacity, but this is still not sufficient for Phase 1.  However, with the exception of the membranes, the City has indicated that there is little of the equipment included as part of the existing pilot plant that they would want to keep long term.  This would mean the purchase and installation of new piping, blowers, instruments, pumps, etc.
• The full-scale pilot has a running horsepower of 63 HP and runs 24/7.  Based on current energy cost of about $0.07/kWh, the annual energy cost is estimated to be $28,800.  Based on the information from the staff about labor time spent with the process and assuming an average labor rate of $40/hr, the annual labor cost is estimated to be $29,100.  There are not associated chemical cost.  The total operating cost is estimated to be $57,900.
• Based on information provided by Enviroquip, the annual membrane replacement cost is estimated to be $1,600 and the annual equipment maintenance cost is estimated to be $4,300, for a total maintenance cost of $5,900.
• The total O&M cost for the current full-scale pilot unit is estimated to be $63,800.  Since the blowers are already sized for 50% more capacity and make up 60% of the power requirements, the energy use is estimated to increase about 60% over current levels to an estimated annual cost of about $46,000.  Operations labor would likely not increase significantly, but maintenance would go up proportionally to $11,800.  Estimated annual O&M cost for the larger process is about $87,000.
• Permanent installation of the membrane thickener would represent a significant cost.  The pilot study report estimated a cost of $688k (before markups).  This was based on the purchase price of the membrane thickener, new sludge pumps, installation, building expansion, piping modifications, and electrical/instrumentation.  The City has already paid for the cost of the membrane thickener through lease payments, but as mentioned earlier, the City would replace all equipment but the membranes.  The membranes could be installed directly into the digester tanks to avoid purchase of new tanks.  Therefore, the main equipment purchase would be two feed pumps, two permeate pumps, two recycle pumps, and two blowers.  A cost of $100k will be assumed for the price of this equipment with installation.  The existing sludge pumps are to be reused, so this cost is no longer included.  The building expansion was estimated at $65k.  Piping modifications was estimated at $60k.  The subtotal is $225k.  Electrical/instrumentation will be assumed at 15% of the subtotal for a total of about $260k (without markups).

Financial Evaluation:

Added Capital Cost w/ Thickening:              $260,000 – Membrane Thickener Installation

                                                                    $205,000 – Additional concrete work for smaller digesters

Deducted Capital Cost w/ Thickening:      ($180,000) – One less dewatering fan press unit

                                                                    ($295,000) – Smaller digesters

Capital Cost Differential w/ Thickening:     ($10,000)

Added Annual O&M Cost w/ Thickening:         $87,000 – Power, labor, maintenance

Deduct Annual O&M Cost w/ Thickening:      ($10,000) – Reduced polymer consumption

                                                                        ($50,000) – Reduced haul cost

                                                                        ($2,000) – Reduced dewatering equipment power and maintenance cost

Annual O&M Cost Diff. w/ Thickening:              $25,000

10-year life cycle cost differential:                    $215,000 assuming 2% effective interest rate (interest minus inflation)

Conclusion:

There are certainly benefits that can be realized with the inclusion of thickening.  However, based on our evaluation, thickening is not a cost-effective process for this application.  There is minimal capital cost savings and the higher O&M costs associated with thickening make the 10-year life cycle cost $215k more than without thickening.  For these reasons, K/J continues to recommend that thickening not be included as part of Phase 1.  The City may wish to revisit this decision prior to implementation of Phase 2.

Tom Giese