Tom –

 

Thank you for that thorough evaluation.  We will stay with our earlier decision of excluding sludge thickening.

 

Jim

 

 

 

 

Jim,

 

Below are our responses to your e-mail:

 

1)  I actually just met with Dennis Gleason this morning on another matter and he brought up the subject of thickening at the Arlington WWTP.  Most of what he describes in his e-mail sounds accurate, but there are a few additional things to point out:

A)        We definitely do not want to send permeate from an MBT to the effluent.  Coming from the digester, that permeate will have higher levels of orthophosphate, due to the destruction of volatile solids and release of P in the digester.  The permeate would need to be recycled to the head of the plant to remove the orthophosphate in the biological process.

B)        The existing MBT has been running at between 2 and 2.5% solids.  We did test at 3% for a short period of time successfully, but due to limitations of the existing system, the system has been unable to run at 3% for an extended period.  Therefore, we’d be hesitant to design an MBT for 3% solids.

C)        Our initial evaluation of digestion with and without thickening in the Schematic Design Report shows that the cost is expected to be less for digestion without thickening, compared to with thickening.  This evaluation was based on constructing new digester tanks.  Now that one existing SBR tank will be used for digestion, there is much less cost associated with the tanks, meaning digestion with thickening would be even more expensive, since sufficient tankage is already available without thickening.

D)        Thickening sludge prior to dewatering will result in some amount of improved dewatering performance.  However, if the digester volume were to be the same for both with and without thickening, the thickened sludge would have a longer SRT, which would actually decrease the dewaterability some, and potentially offset benefits of dewatering a thicker sludge.

E)        Having dewatering means having to operate, maintain, monitor, and clean another unit process.

F)         A potential benefit of thickening are that only a portion of the existing SBR tank would need to be converted to digestion.  What wasn’t needed could be left empty.  This wouldn’t reduce construction cost, because you’d still need to build a wall to partition the tank.  It may reduce the cover costs, if instead you put a drain in the unused portion of the tank to keep it from collecting water as opposed to covering it.  The equipment cost would be the same, because we’d be designing for the same SRT, which would require the same air for digestion and the same size blowers.  The only other significant potential benefit is that thicker sludge would improve performance of the dewatering process, assuming operation at the same SRT.  The question then is whether or not the improvement in performance warrants the capital and O&M cost of a thickener.

G)        The capital cost we allocated for the membrane thickener originally is $800,000.  The estimated annual O&M cost for the thickener is $62,500.

 

2)  I couldn’t figure out where you pulled the numbers in your table below from, but based on those numbers I would note a few things:

A)        Although the thicker sludge shows a better cake concentration, there is not a big difference (though still significant) and the results with the 1% sludge is very close to the target of 15%.

B)        If I’m reading the polymer feed rates right, it appears that the amount of polymer added for the thicker sludge (ratio of 0.75 parts polymer per 4.5 parts sludge??) was higher than for the thinner sludge (ratio of 0.66 parts polymer per 14.5 parts sludge??).  Therefore, it would be difficult to say whether or not it was the higher polymer dose or thicker sludge, or both that contributed to the higher cake solids.

C)        Based on our discussions with the manufacturers and talking to a number of their references, as documented in the Solids Handling Evaluation Tech Memo, it appears feasible that you could dewater 1% WAS to 15% cake solids.

D)        As Dennis mentioned below, the FKC screw press does generally have a lower solids capture compared to a rotary fan press, although Huber claims their screw press capture rate is inline with a rotary fan press.

E)        Given the capital and O&M cost associated with the membrane thickener, it may be less expensive to use more polymer to increase dewaterability of the thinner sludge and/or pay more money to haul the larger volume of sludge (about 14% more based on the numbers in your table below).

 

3)  Redlands normally operated at 7,000 to 8,000 mg/l.  Martin Way operates at around 8,000 mg/l.  Hyrum operates around 10,000 mg/l.  Karcher Creek also normally operates around 10,000 mg/l, as does the Kamilche WRP on the Squaxin Island Reservation.  At startup, the WWTP would be operating at between 6,000 and 7,000 mg/l if all aeration basin trains were in operation.  It may be beneficial early on from an efficiency and performance standpoint to operate only two aeration basins, to achieve a higher MLSS.  Consideration can also be given to increasing the SRT to raise the MLSS value, although as mentioned above an increased SRT could offset benefits of having thicker sludge when it comes to dewatering.  As discussed in the attached e-mail I sent a while back, the plants that are running below 10,000 mg/l are largely doing so out of choice rather than necessity.  There are means to increase the MLSS if desired and beneficial overall.

 

4)  Glad to hear you’ve setup a pilot test for a rotary fan press.  I think this will help greatly in making the selection between a screw press or fan press.

 

If the City would like to revisit the earlier decision to exclude sludge thickening, then we should do so.  It is important that all involved are comfortable with the direction the design is headed.  However, without precise evaluation, it appears that adding thickening back into the project would increase the O&M cost, as well as the capital cost.

 

 

Chris & Tom –

 

1. Below is an email from Dennis Gleason regarding sludge thickening, this email was generated from my inquiry to Jim Gleason regarding COA receiving credit for payments made to rent the Enviroquip MBT.  Your thoughts on his suggestions about thickening?

 

2. Regarding sludge thickening and dewaterability, below is a quick snapshot of the data from the FKC screw press pilot study:

 

FKC Screw Press Pilot Study
Sludge %	Cake %	Poly feed Rate (mm/min)
1.92%	16.50%	0.75 poly:4.5 sludge
1.00%	14.50%	0.66 poly:14.5 sludge

 

 

3. David said that he thinks the MBR plants we visited had sludge in the 0.7% range, any information from your notes.

 

4. David is working with Dennis to get a pilot study of the rotary fan press for the COA plant – no cost.

 

 

With the high estimated costs of the BCF, we are now looking at alternate means for sludge handling.  Do you think this issue (Sludge Thickening) is something we should put on the agenda for the upcoming design workshop.

 

Jim

 

 

Jim,

 

To reiterate our phone discussion earlier today, based on my knowledge of the screwpress vs rotary press.

 

Neither unit does well with solids concentrations in the 1% range.  So thickening would be required.

 

Assume 100,000gpd @ 1% for both systems

 

Membrane thickener to 3%.  The filtrate can go to the effluent  (67,000gallons).  It is the same as the effluent from the treatment process.  All the solids wasted are still in the sludge holding tank since the capture rate will be 100%.

 

Drum Thickener to 3%.  The filtrate needs to be returned to the MBR since it has about 800+ lbs of waste solids (assume 90% capture rate).  Therefore in order to waste the 8340#’s of solids in the above assumption you will need to waste an additional 10,000 gallons every day. 

 

Next to dewatering:

 

FKC screwpress: Capture rate will be about 85% so,  33,000gallons with 8260#’s of solids will equal 7018#’s dewatered to 15% (guess).  So, again another 1240#’s of solids go back to the MBR with the 27,400 gallons of filtrate.

 

Fournier:  Capture rate will be about 93% so, 33,000gallons with 8340#’s of solids will equal 7756#’s  dewatered to 15% (guess).  So, now the filtrate of 27,390gallons can go to the sludge holding tank since a membrane thickener is in the loop there.

 

On to polymer usage:  There is none needed for the membrane thickener.  Assume 15#/Dry ton for the drum thickener. @ 4dt/day that will be 60#/day @ $3.00/# or $180/day  (5x50x$180=$45,000/yr)

 

Secondary benefit is the sludge storage tank sizing.  If the desire is to have a 10 day storage then with the drum thickener you will need a 1MG tank.

 

With the membrane thickener a .333mg tank will be adequate. 

 

The only downside to the membrane thickener is the blower usage, but with the storage tank size decreasing you will only need 1/3^rd the air to keep it mixed as you would with the 1mg tank which should be comparable to the amount of air needed for the membrane thickener.

 

Please call me to discuss this further…it may not be totally clear.

 

Also, should I send on to Tom Giese?

 

 

 

                                                                                                             

Dennis Gleason

Treatment Equipment Company

Office: 425-641-4306

Mobile: 425-681-7015

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