Special Report MUB WWTP Expansion and Upgrade Evaluation of Alternatives

Transcription

1 Special Report MUB WWTP Expansion and Upgrade Evaluation of Alternatives Introduction The expansion and upgrade of the MUB Wastewater Treatment Plant (WWTP) is a critical component of MUB s response to Federal policy related to the management of Combined Sewer Overflows (CSO s). Federal CSO policy requires CSO communities to prepare and implement Long Term Control Plans (LTCP s) in pursuit of the goal of significantly reducing / minimizing CSO discharges. Our current WWTP capacity is 12 million gallons per day (mgd). The average daily flow to the plant in 2014 was 11.1 mgd. The average daily flow in 2011 (an especially rainy year) was 11.7 mgd. Clearly, the current demands on the WWTP will soon exceed the plant capacity. In addition to the above, 1/3 of the current WWTP capacity is provided by obsolete equipment, known as Rotating Biological Contactors (RBC s). The RBC s are now 35 years old, and 10 years beyond their useful life. They often fail, and replacement parts are increasingly expensive and difficult to obtain. MUB proposes to increase WWTP capacity from 12.0 to 20.8 mgd. This will be accomplished by replacing the RBC s with Membrane Bio Reactors (MBR s). The added capacity will address each of the challenges described above, and will provide additional capacity to serve the needs of our community s growing population, and all within the constraints of our current plant site. Other upgrades will include expansion of the headworks and digestion processes (which are also operating at maximum capacity), replacement of out of date electrical gear, and implementation of ultraviolet () disinfection. Comparison of Alternatives This report provides a concise summary of very detailed analyses performed by Strand Associates and MUB staff over the period of 2013 to Those analyses focused primarily on the secondary treatment component of the WWTP, and on the digesters. Each alternative was evaluated for its operational capability at a proposed plant capacity of 20.8 mgd, as follows: 1

2 Biological Treatment Options Secondary treatment is a biological process aimed at removing fine and dissolved organics from the wastewater, after it has first been screened and settled (by primary treatment) to remove larger and more coarse particles. Secondary treatment is typically accomplished by using populations of micro-organisms to consume the organics. The current MUB WWTP accomplishes that by use of the obsolete RBC s (for 4 mgd of plant capacity), and by the use of Activated Sludge (for 8 mgd of plant capacity). The existing Activated Sludge and RBC units operate in parallel. Every option under consideration eliminates and replaces the RBC s. The RBC process will not be described further, due to its obsolescence. Activated Sludge utilizes aeration basins in which a concentrated liquor of micro-organisms resides. Wastewater is fed into the basins, and the micro-organisms feed upon the organics from the waste stream. Air is continuously blown into the basins both to sustain the micro-organisms (so they may breathe ) and to keep the liquor mixed. Following this process, the wastewater flows to large circular clarifiers, where the micro-organisms and remaining organics are allowed to settle. A portion of the settled organics are re-cycled to the aeration basins to maintain the desired liquor concentration, and the excess remaining organics flow to the digesters for further processing. Long List: A total of six biological systems / configurations were considered. The results of the analyses are summarized in the attached Exhibit 1; the selected systems are indicated in yellow. Three of the six systems were eliminated from consideration in the first round of analysis. Those were: BT3 Integrated fixed film activated sludge and moving bed biofilm reactor. This system was rejected because its fixed film technology too closely resembled that of the RBC s. It depends upon maintaining a fixed film of microorganisms on a surface which is submerged in the wastewater. From our experience with the RBC s we know that this technology is prone to operational problems (clumping), and does not provide the flexibility needed for flows which vary dramatically with rainfall. 2

3 BT4 - Sequential Batch Reactor. This system was rejected because its technology is better suited to smaller capacity plants. The SBR alternates two functions in a single vessel (reactor). It first aerates much like activated sludge, then switches to clarification within the same vessel by simply turning the aeration off. This sequencing requires multiple SBR s in parallel, so that some are aerating while others are clarifying at any given moment. Larger capacity plants (like ours) require a larger number of reactor vessels in order to ensure continuous flow. Even so, this system does not lend itself well to flows which vary dramatically with rainfall. BT5 Bio Mag Ballast Settling System. This system is a modified activated sludge system, which utilizes magnetite as a flocculent, and which claims space saving efficiencies due to increased liquor concentrations and faster settling times. While promising, this technology has only recently been introduced, and lacks sufficient service history to justify consideration. In addition, there are concerns that use of the magnetite ballast might introduce excessive operational management demands on the staff. Short List: The remaining three systems considered were evaluated in detail. In addition to operational capability, these alternatives were evaluated on the basis of their land space requirement and their future expandability. The three short list options were: BT1 Full Activated Sludge Plant BT2 Full Membrane Plant BT6 Existing Activated Sludge Plant with Membrane Addition Discussion of these 3 alternative is as follows: BT1 Full Activated Sludge Plant. The Activated Sludge portion of the existing WWTP operates very well under all conditions. This option would expand and upgrade the WWTP by adding more Activated Sludge aeration and clarification units, using the same design criteria as was used for the existing plant. Advantages: Provides high confidence through proven technology Simplified operations relying on a single process technology 3

4 Disadvantages: Requires relatively large amount of land space per unit of capacity, to the degree that the existing site would be fully consumed at the current design capacity of 20.8 mgd, and future expansion (planned to be 27.7 mgd) will require acquisition of additional land (84 Lumber site, or similar). Requires relatively large additional structures for future capacity additions, making future plant upgrades both significantly less flexible and more expensive than other alternatives. Cost: $79.8 million BT2 Full Membrane Plant. This option relies upon Membrane Bio Reactor (MBR) technology. An MBR is much like activated sludge, in that it uses very similar aeration basins but higher mixed liquor concentrations. The chief difference is that MBR s eliminate the secondary clarifiers, by immersing a membrane filter into the treated mixed liquor. The membrane filters physically separate the solids from the wastewater, so that clarification by settling is not necessary. This option would expand and upgrade the WWTP by replacing the Activated Sludge aeration and clarification units, with MBR s of suitable capacity. Advantages: Provides high confidence through proven technology Simplified operations relying on a single process technology Efficient use of land space per unit of capacity, to the degree that the existing site will accommodate both the current design capacity of 20.8 mgd, and future expansion (planned to be 27.7 mgd) without acquisition of additional land. Allows future capacity additions to be made in smaller increments, making future plant upgrades both significantly more flexible and less expensive than other alternatives. Disadvantages: Requires elimination and demolition of existing Activated Sludge system, despite its relatively new condition and excellent performance. Requires adjustments to operations relying on a new process technology Cost: $80.8 million

5 BT6 Existing Activated Sludge Plant with Membrane Addition. This option retains the existing Activated Sludge plant (with 8.0 mgd capacity) and supplements it with Membrane Bio Reactor (MBR) technology (with 12.8 mgd capacity). See above for a description of MBR technology. Advantages: Provides high confidence through proven technology Efficient use of land space per unit of capacity, to the degree that the existing site will accommodate both the current design capacity of 20.8 mgd, and future expansion (planned to be 27.7 mgd) without acquisition of additional land. Allows future MBR capacity additions to be made in smaller increments, making future plant upgrades both significantly more flexible and less expensive than other alternatives. Avoids elimination and demolition of existing Activated Sludge system, and preserves its relatively new condition and excellent performance. This correspondingly reduces the amount of MBR capacity that must be built. Disadvantages: Requires adjustments to operations relying on a new process technology Operations rely upon multiple (dual) process technologies Cost: $76.6 million Digester Options Digestion is a combined physical and biological process which concentrates sludge and reduces its volatile organic content. We currently have two identical digester units. They are operating at maximum capacity. A third digester is proposed, with sufficient capacity corresponding to that of the proposed MBR units. Due to space constraints on the site, each of the digester options provide added capacity through heights taller than the existing units. Two types of digesters were considered. Those were: 5

6 A-ESD Egg Shaped Steel Digester. The egg shaped digester presents an aesthetically appealing appearance, but at the cost of an expensive proprietary design. The egg shape offers some marginally better control of foaming (due to a smaller exposed surface area), but not enough advantage to offset the higher proprietary cost. Advantages: State of the art technology, with aesthetically pleasing appearance Lower potential for foaming Disadvantages: Proprietary design (translates into disproportionate high cost vs benefit) Cost: $12.0 million B-Silo Cylindrical Silo Digester. The cylindrical digester is functionally equal to the egg shaped alternative, differing only in its shape and material of construction (concrete for the cylindrical shape versus steel for the egg shape). Advantages: Conventional technology Non - Proprietary design (translates into better cost vs benefit) Disadvantages: Slightly higher foaming potential (but no worse than existing units) Cost: $ 8.9 million 6

7 Poponoe Run Interceptor Replacement The age, condition, and capacity of the existing box culvert interceptor combine to dictate that it be must be replaced from lower Krepps Park to Brewer Street (in Star City). There are no practical alternatives to be considered. The location of the existing Interceptor is dictated by gravity, and follows the path of the Poponoe Run stream. Any alternate route would require pumping or tunneling, and to such a degree that the higher costs of such alternatives are prohibitive by inspection. Alternatives in material selection have been explored, but are all more/less the same cost, making comparison here irrelevant. Cost of this pipeline is estimated at $4.5 million. Summary For the major component of the WWTP expansion, we chose the option of using existing activated sludge, supplemented by construction of a parallel membrane bio reactor process for the additional needed capacity. This option preserves our previous investment, and excellent current performance of the activated sludge units. In addition, the MBR addition will make efficient use of our limited land space, and will allow future expansions to be done in smaller and more economical increments. This is also the least expensive of the three short list alternatives. The choice of digesters was straight forward. The two alternatives are functional equals, and we simply chose the cylindrical option because it is less expensive. The Poponoe Run Interceptor was even simpler, as no practical alternatives are available. The chosen sewer project components consists of: Existing Activated Sludge with Membrane Addition Cylindrical Digester Poponoe Run Interceptor Replacement $ 76.6 million $ 8.9 million $ 4.5 million Total $ 90.0 million 7

8 Exhibit 1 Biological Treatment Options Option Description Comment Construction Design Future Million $ Capacity Capacity BT1 Full Activated Sludge Plant $ mgd mgd BT2 Full Membrane Plant $ mgd mgd Eliminated due to technology BT3 Integrated Fixed-Film Activated being too similar to existing Sludge and Moving Bed and obsolete Rotating Biofilm Reactor Biological Contactors Eliminated due to difficulty BT4 Sequential Batch Reactor controlling the timing with larger systems Eliminated due to technology BT5 BioMag Ballast Settling System and limited installations to establish a proven record BT6 Existing Activated Sludge With $ Parallel Membrane Addition mgd mgd Digester Options 1A-ESD Egg Shaped Steel Digester $12.0 1B-Silo Cylindrical Silo Digester $8.9 Poponoe Run Box Culvert Replacment Poponoe Run - Krepps Park $4.5 Total Estimated Cost for Selected Components $90.0

9 INFLUENT SPLITTER PRIMARY CLARIFIERS AERATION TANK (NEW) AERATION TANK (EXISTING) FINAL CLARIFIERS (3 EXISTING AND 2 NEW) DISINFECTION AND EFFLUENT PUMPING MGD PC 1 PC PC MGD 6.9 MGD MGD MGD MGD MGD

10 Future Final Clarifier (95-ft diameter) Final Clarifiers (90-ft diameter) Three 36 MLSS Pipes (to Final Clarifier No. 4, 5, and 6) Aeration Tanks (4 tanks 28 ft x 185 ft x 20 ft) MLSS Splitter for New Final Clarifiers Disinfection, Effluent Pumping, and Outfall (50 ft x 45 ft) RAS Manholes and Pumps Piping Legend Primary Effluent MLSS RAS Secondary Effluent Final Effluent

11 INFLUENT SPLITTER PRIMARY CLARIFIERS INTERMEDIATE PUMPING STATION MBR TANK AERATION TANK (EXISTING) 99.7 MGD 79 MGD MGD PC 1 PC MGD MGD PC MGD MGD MGD DISINFECTION

12 New Primary Effluent Splitter Box Ancillary Equipment for MBRs (36 ft x 80 ft) MBR Tanks (9 ft x 82 ft) Zones New Pumping Station and Fine Screens (38 ft x 86 ft) New Disinfection and Outfall (50 ft x 30 ft) Piping Legend Primary Effluent MLSS RAS Final Effluent

13 INFLUENT SPLITTER PRIMARY CLARIFIERS INTERMEDIATE PUMPING STATION AERATION TANK (NEW) MBR TANK AERATION TANK (EXISTING) 53.3 MGD 42.2 MGD MGD RAS From Final Clarifiers PC 1 PC MGD MGD PC MGD 11.1 MGD 9.68 MGD 9.68 MGD 9.68 MGD 42 Secondary Effluent From Final Clarifiers MGD MLSS To Final Clarifiers MGD DISINFECTION AND EFFLUENT PUMPING

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