TECHNICAL MEMORANDUM

TO: Phillip Bowman, P. E., City Engineer FROM: Robert Mayers, P. E.

Jeff Beckman, P. E. DATE: October 27, 2016 SUBJECT: Existing Moab WWTP Capacity Evaluation JOB NO.: 130-16-05

EXECUTIVE SUM M ARY Bowen Collins & Associates (BC&A) has been requested by the City of Moab to complete a preliminary evaluation of the available capacity at the existing city wastewater treatment facility (WWTP). The purpose of this this technical memorandum is to summarize the estimated available capacity of the existing Moab WWTP. This Executive Summary section provides a brief summary of the evaluation, and additional detail is provided in the subsequent sections. The Moab WWTP is a single-stage trickling filter plant with primary and secondary digesters along with chlorine disinfection. The WWTP has not been able to consistently meet the water quality standards established by the Utah Department of Environmental Quality (UDEQ). Among other requirements, the established standards limit Biochemical Oxygen Demand (BOD5) and Total Suspended Solids (TSS) in the treated effluent to 25 mg/ l. BOD5 and TSS are the common cause for permit violations at the Moab WWTP. Figure ES-1 summarizes monthly averages for BOD5 and TSS concentrations in the Moab WWTP effluent from January 2012 through August 2016. Upon review of the design criteria that is included in the construction drawings, it was determined that the design criteria for the Moab WWTP design is w ithin common design practices for this type of treatment facility. The estimated design capacity for the influent load, for both BOD5 and TSS is 3,300 LB/ Day in the warmer months and 2,170 LB/ Day in the winter months. Figure ES-2 compares the historical influent loading with the design capacity. Figure ES-2 shows that the influent loading is at or exceeds design capacity for most summer months. H istorical data indicates that the existing treatment plant cannot reliably meet the high influent loading. It is our opinion that no additional treatment capacity is available at the existing WWTP. Enhanced performance achieved by one or more of the recommended measures may reduce the frequency of violations; but unless and until that can be reliably demonstrated, there is no ability present to treat any additional organic loads (BOD5 and TSS) at the Moab wastewater treatment plant.

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EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 2

BACKGROUND Bowen Collins & Associates (BC&A) has been requested by the City of Moab to provide a technical memorandum regarding the capacity of the existing city wastewater treatment facility (WWTP). The WWTP was initially constructed in 1959 and has been expanded and upgraded tw ice since then. It is a single-stage trickling filter plant w ith primary and secondary clarifiers and chlorine effluent disinfection, a typical process configuration for the time. Anaerobic digesters and drying beds were provided for treating waste bio-solids, w ith disposal of this material by landfill ing. Treated effluent is discharged to the Colorado River following disinfection. For a period of years, effluent from the WWTP has experienced periodic violations of water quality standards established by the Utah Department of Environmental Quality. These violations bring into question the capability of the plant to treat current or additional future flows while meeting these standards. The violations may be due to design, construction or capacity inadequacies related to current wastewater loadings, and/ or to operational procedures. Accelerated growth in housing and other construction is occurring in Moab which adds to the wastewater flows and increases treatment requirements at the WWTP. A new treatment facility is currently under design to replace the existing WWTP and is anticipated to be operational in 18-20 months. The new plant will provide increased capacity to accommodate future growth and improved treatment performance to meet effluent discharge quality requirements. However, for the interim period prior to completing that project, concern has been raised regarding whether the existing WWTP can accommodate any additional flows and loads without increased permit violations. EXISTING FACILITY Figure 1 is a Location Map for the WWTP situated in the northwestern part of the community in southeastern Utah. Figure 2 provides the Site Plan for the existing plant w ith various major elements indicated. Figure 3 presents a Simplified Process Schematic Diagram. Note that the original anaerobic digesters shown on the diagram have not been functional for several years and currently serve only as holding tanks prior to dewatering. Temporary mechanical belt filter press dewatering equipment has been installed to supplement the performance of the drying beds which were performing poorly. Table 1 summarizes selected design criteria for elements of the WWTP taken from construction drawings for a plant expansion project dated 1997. This information focuses on processes that specifically provide the physical and biological wastewater treatment capability to remove biochemical oxygen demand (BOD5) and total suspended solids (TSS) from the flow prior to discharge. The primary clarifiers, trickling filters and final clarifiers are listed, plus overall plant flows and loads. Important but unrelated aspects to this evaluation including grit removal, screening, solids dewatering and disposal, disinfection, pumping, hydraulic capacity, etc. are not addressed in this technical memorandum. However, it is noted that the facility does provide the needed pumping and hydraulic capacity to convey the flows associated with loads of BOD5 and TSS being considered.

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EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 3

Table 1 Existing M oab WWTP Plant Information

I tem Units Size or Capacity Plant Hydraulic Capacity

• Peak Month Average Day • Peak Hour

MGD MGD

1.5 3.1

Plant Organic Capacity (Peak Month Average Day) • BOD5 • TSS

LB/ Day LB/ Day

3,300 3,300

Primary Clarifier Nos. 1, 2 • Diameter --- Each Clarifier • Side Wall Depth • Influent Well Diameter • Weir Length --- Each Clarifier • Weir Loading • Surface Area --- Each Clarifier • Surface Loading • Volume • Detention Time

FT FT FT LF

GPD/ LF SF

GPD/ SF CF HR

40 7, 8 6, 8 126

5,950 1,256 600

8,790, 10,050 2.1, 2.4

Trickling Filter Nos. 1, 2 (Cobble Rock Media)

• Diameters • Media Depth --- Each Filter • Total Media Area • Total Media Volume • Total BOD5 Feed --- Summer • Total BOD5 Feed --- Winter • Total BOD5 Volumetric Loading Rate --- Summer • Total BOD5 Volumetric Loading Rate --- Winter • Total Hydraulic Feed --- Summer • Total Hydraulic Feed --- Winter • Total Hydraulic Surface Loading Rate ---

Summer • Total Hydraulic Surface Loading Rate --- Winter • Recycle Ratio --- Summer • Recycle Ration --- Winter • Hydraulic Design Intensity --- Summer/ Winter • Distributor Speed at HDI --- Summer • Distributor Speed at HDI --- Winter • Hydraulic Flushing Intensity --- Summer/ Winter • Distributor Speed at HFI

FT FT

SF, AC CF

LB/ Day LB/ Day

LB/ 1000 CF/ Day LB/ 1000 CF/ Day

MGD MGD

MGD/ AC MGD/ AC

-- --

IN/ Pass MIN/ REV IN/ Pass

MIN/ REV

72, 80 7

9,090, 0.209 63,600 2,200 1,070 34.5 16.8 3.74 2.01 17.9 9.6 1.5 1.26 0.40 1.7 3.2 6.0 26.2

Final Clarifier No. 1 • Diameter • Side Wall Depth • Influent Well Diameter • Weir Length

FT FT FT

40 7 6

EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 4

• Weir Loading • Surface Area • Surface Loading Rate • Volume • Detention Time • Tube Settlers

LF GPD/ LF

SF GPD/ SF

CF HR --

242 3,100 1,256 600

8,790 2.1 Yes

Final Clarifier No. 2 • Diameter • Side Wall Depth • Influent Well Diameter • Weir Length • Weir Loading • Surface Area • Surface Loading • Volume • Detention Time • Tube Settlers

FT FT FT LF

GPD/ LF SF

GPD/ SF CF HR --

40 10 11 126

3,690 1,256 370

12,560 4.8 No

The Moab WWTP processes are capable of removing BOD5 and TSS from the wastewater, and may also reduce some ammonia (nitrification) depending on loading rates, temperatures, etc. However, this system is inherently incapable of reducing nitrate concentrations (denitrification) or of achieving low phosphorus amounts in the effluent. Table 2 provides some additional design information derived from the information in Table 1.

Table 2 Additional Design Information

Parameter Raw Wastewater BOD5 Concentration 264 MG/L Raw Wastewater TSS Concentration 264 MG/L Primary Clarifier Capacity, Each 0.75 MGD BOD5 Removal Rate in Primary Clarifiers 33% Final Clarifier Capacity in No. 1 0.75 MGD Final Clarifier Capacity in No. 2 0.46 MGD

Table 3 contains the treated effluent discharge standards for the Moab WWTP. The BOD5 and TSS 30-day average concentrations of 25 MG/ L each and 85% removal each are significant for this evaluation.

EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 5

Table 3 Effluent Discharge Permit Requirements for Discharge to Colorado River

Parameter Max. Monthly Ave. Max. Weekly Ave.

Daily Min.

Daily Max.

BOD5 Concentration Removal Rate

25 MG/L 85% Removal

35 MG/L - -

TSS Concentration Removal Rate

25 MG/L 85% Removal

35 MG/L - -

E-coli 126/100 ML 157/100 ML - - TRC 1.4 MG/L 1.55 MG/L - - WET, Acute - - - LC50 >100% effluent Oil & Grease - - - 10 MG/L pH - - 6.5 units 9.0 units TDS <400 MG/L increase - - -

Based on the hydraulic loading rate, BOD5 loading rate and the recycle ratio information for the trickling filters, they are classified as single stage, high rate units. A removal capacity of 65-85% of primary effluent BOD5 applied is typical for these type of filters. For warm weather operation, the existing plant is designed to treat up 1.5 MGD peak monthly flowrate at 264 MG/ L and 3,300 LB/ Day influent BOD5 and TSS, and achieve 85% or more removal of these constituents and a maximum 30 day average effluent concentrations of each of 25 MG/ L. For cold weather operation, the capacity of the trickling filters is reduced to 1,070 LB/ Day BOD5, a decrease of 51%. Trickling filter performance depends on wastewater temperatures which affect the activity and population of microorganisms present in the process that metabolize organic constituents present in the flow. The reduced capacity figure appears to be very conservative, as a 30% reduction is more in line with experience and textbook information. However, trickling filter plants that practice recirculation such as the Moab WWTP may see further reductions in performance as a result of passing more of the lower temperature flow back through the filters. Therefore, this evaluation will consider the value as given to govern cold weather operation, although actual performance may exceed it. HISTORICAL TREATM ENT PERFORM ANCE AND CURRENT CONDITIONS As indicated above, the Moab WWTP has been operating continuously since 1959 treating municipal wastewater from the community and discharging to the Colorado River. But as flows and loads have increased over the years, effluent quality has decreased and during periods the WWTP has had difficulty meeting requirements of its discharge permit. Figure 4 shows monthly average plant effluent quality monitoring data from January 2012 through July 2016. Changes in wastewater flows and loads during this period generally reflect growth in resident population, visitation and business activity that occurred in Moab. Figure 5 shows influent BOD5 and TSS loads for January 2012 through July 2016 at the Moab WWTP and compares them to plant design capacity limits. It should be noted that the plant receives substantial quantities of septage from haulers that serve nearby national and state parks, campgrounds, isolated homes and other facilities. Influent sampling at the WWTP for influent has typically been performed at times when the impacts of septage loads are not captured. As a result, influent BOD5 and TSS influent values are under-

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EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 6

reported. Analyses performed for the Facilities Master Plan determined that approximately 304 LBS of BOD5 are contained in average strength 2800 gallon septage truck load, and that up to 10 loads per week may be delivered to the plant in the summer months. This equates to an additional 434 LBS/ Day BOD5 in the summer months. Winter months are estimated at one low strength truck per day that equals approximately 91 LBS/ Day of BOD5 during the winter period. The BOD5 and TSS loads shown in Figure 5 include the estimated additional septage. The total loading indicates influent loading capacity exceedances occur nearly every season. Data for the current year 2016 does not indicate a capacity exceedance, but approaches the ultimate treatment capacity. However, any time that loads begin to approach the design capability of the plant, the opportunity for permit violations increases significantly. Loads for the winter periods do not exceed the indicated cold weather capacity. DISCUSSION Organic loads regularly exceed plant design capacity limits regularly in the summer months, and the plant runs near, at or above capacity for the entire season. This condition and occurrence is the most likely reason that the existing plant does not continuously meet the discharge requirements. Other potential reasons for sporadic treatment performance include design/ construction deficiencies, the aged condition of the facility, and operational practices. There are no obvious deficits in the design or construction of the existing facility, and this is not believed to be a contributing factor in plant performance. The condition of the rock media, concrete surfaces, underdrains, piping, etc. in the trickling filters is not known, but due to age and exposure to aggressive conditions, it is possible that degradation of these and other items has occurred which could affect the performance of these and other treatment units. However, it would be necessary to remove the filters from service and perform at least partial deconstruction to determine those conditions. Such an undertaking would be costly and time consuming, and result in substantially reduced treatment capacity w ith even one unit out of service for a lengthy time. It has been suggested that the sludge collection mechanisms in the final clarifiers may be in poor condition and in need of repair or replacement, possibly due to damage from concrete and rock debris shed from the trickling filters upstream as those materials deteriorate. It should be possible to remove one final clarifier at a time from service for a day or more in the winter months to dewater it and expose the mechanisms for inspection. If the mechanisms are damaged but repairable, then repairs would be recommended. Replacement would be more time consuming and costly, and they are expected to operate for only another 18-20 months. This decision would be based on conditions noted at that time. Operational practices that are available to help improve trickling filter operation, and thus overall plant performance, include recirculation of effluent back onto the filters to improve wetting and distribution and reduce the effective organic concentration in the primary effluent being applied. The capability to perform this procedure exists at the Moab WWTP and the plant operator reports that it is practiced regularly. It may be effective to increase or reduce recirculation rates in

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EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 7

summer and winter months to achieve improved performance, and it is recommended that these adjustments be implemented in attempt to maximize the existing capacity of the trickling filters. The filters are also equipped with mechanisms to enable the flow-driven distribution arms to be slowed down in order to increase the intensity and effect of flushing on the rock media. Increased flushing helps remove aged biofilm from the media and expose new, younger and more rapidly growing microorganisms with higher metabolic activity to the primary effluent. This process should increase the uptake of organic constituents from the wastewater and conversion to biomass, and provide improved filter performance. The plant operator reports that this procedure is practiced, but it may be effective for it to be done on a more regular and/ or intense basis. It is widespread common practice to add a coagulant such as ferric chloride either to the primary clarifiers for increased BOD5 removal prior to the trickling filters, or to the secondary clarifiers to improve their removal performance prior to effluent discharge. This practice was recommended in a previous memo and has been implemented at the Moab WWTP. The plant operator has reported limited success or improvements and/ or related operational difficulties and has discontinued this effort to our knowledge. However, we believe that this practice should provide the desired improvements, albeit possibly at greater dosages than have been attempted thus far. The above investigation and operational changes are recommended in attempt to reduce the magnitude and frequency of effluent permit violations and improve the functioning of the existing WWTP for an interim period until the new plant is constructed, which is expected to occur w ithin approximately 18-20 months. However successful these efforts might prove, they do not change the influent wastewater loadings to the plant and which indicate that the facility operates at or above its design capabilities for much of the time. CONCLUSION It is our opinion that no additional treatment capacity is available at the existing WWTP. H istorical data indicates that the existing treatment plant cannot reliably meet the high influent loading. Enhanced performance achieved by the above recommended measures may reduce violations; but unless and until that can be reliably demonstrated, there is no ability present to treat any additional organic loads (BOD5 and TSS) at the Moab wastewater treatment plant.

EXISTING MOAB WWTP CAPACITY EVALUATION

BOWEN COLLINS & ASSOCIATES CITY OF MOAB, UTAH 8

GLOSSARY AC Acre Ave. Average BOD5 5-Day biochemical oxygen demand CF Cubic foot E-coli Escherichia coli (organism) FT Foot GPD/ LF Gallons per day per lineal foot GPD/ SF Gallons per day per square foot HDI Hydraulic design intensity HFI Hydraulic flushing intensity HR Hour IN/ Pass Inches per pass LB/ Day Pound per day LB/ 1000 CF/ Day Pounds per 1000 cubic foot per day LF Lineal foot LC50 > 100% effluent Lethal concentration to 50% or more test organisms in 100% treated

effluent environment MGD Million gallons per day MGD/ AC Million gallons per day per acre MG/ L Milligrams per liter MIN/ REV Minutes per revolution ML Milli l iter No. Number Nos. Numbers pH Negative logarithm of hydrogen ion concentration SF Square foot TDS Total dissolved solids TN Total nitrogen TP Total phosphorus TRC Total residual chlorine TSS Total suspended solids WET Whole effluent toxicity (test) WWTP Wastewater treatment plant