Comment

Activated Sludge System Design

Response

The step-wise approach excerpted by the reviewer from textbooks on activated sludge and aerobic digestion design are an out-dated, overly simplistic approach to activated sludge, nutrient removal and aerobic digestion process design. Process models have been developed over the last 30 years specifically to better capture the complexity and number of biological, physical and chemical interactions occuring in activated sludge and aerobic digestion systems. The Biowin model is routinely calibrated against data collected from hundreds of treatment plants worldwide. A more comprehensive listing of parameters used in the Biowin process model can be added to the Final Draft Report, but Biowin’s parameters are also available online.

Comment

Basic and crucial treatment process data, such as specific growth rate for nitrifying organisms (µn), and the dependent solids retention time (SRT), are omitted in the MBR analysis.

Response

The power of a modern, state-of-the-art process model is that although nitrifier growth rate is much more complex than the textbook calculations imply, this complexity can be simulated with a computer. For reference, Biowin’s default maximum growth rate for Ammonia Oxidizing Bacteria (AOB, nitrifiers) is 0.9/day. The base rate expression for nitrifier growth process is the product of the maximum specific growth rate, the ammonia oxidizing biomass concentration and a Monod expression for ammonia. This base rate is modified to account for environmental conditions (e.g., zero growth at low dissolved oxygen and inhibited by nitrous acid), nutrient limitations (phosphate, inorganic carbon, other cations and anions) and pH inhibition. The MBR models were run at several different SRTs (12 to 30 days, to capture the range of temperature and loading conditions experienced in Big Sky). One of the primary advantages of MBR technology is that it enables longer-than-usual SRTs to ensure nitrification is complete. This information will be included in the Final Draft.

Comment

For convenience, Appendix B3 of this report contains excerpts from the Big Sky County analysis by AE2S. On the second page of Appendix B3, labeled Big Sky UCT MBR with PostAnox Bio and Chem 16oC, it can be seen that only limited simulated effluent data is presented. For example, the projected facility flow rate is “0.00 mgd”, which cannot provide information as to the projected effluent concentrations. This includes flow associated with nitrogen, phosphorus, ammonia and other effluent parameters theoretically analyzed in the AE2S report. None of the biological and chemical parameters used to derive these effluent concentrations is presented by AE2S.

Response

Full process model inputs and outputs will be included in the Final Draft. The 0.00 mgd error in the reporting should have read 1.4 mgd, which is the maximum month design flow and load. The maximum month flow and load was modeled at 6 and 16 deg C, to reflect that this maximum condition can occur over December-January, or March-April timeframes, depending on occupancy and I/I (infiltration and inflow).

Comment

There is also a risk that with anticipated chemical use (aluminum sulfate) the wastewater treatment sludge potentially adversely affects the sludge stored on the BSCWSD property

Response

Alum should cause no issues with the sludge stored on BSCWSD property. However, to the reviewers point, the effect of the additional aluminum on the compost quality should be considered. It may warrant utilization of a different phosphorus adsorbing coagulant with more beneficial land application qualities (such as ferric-based metal salt coagulants). This will be added to the CDR Final Draft.

Comment

Biosolids (Sludge) Handling: multiple comments regarding biosolids capacity and the District’s ability to handle and compost additional biosolids generation

Response

Several utilities in Montana rely on composting for compliance with 503c standards for Class A Biosolids. The sludge entering the composting process do not have to meet Class B before introduction into the composting vessels. Capacity of the solids handling system is a labor and composting vessel issue. If the District needs to process more biosolids after MBR implementation, then additional staff time and potentially an additional composting vessel or two will be needed. This is described in Technical Memorandum No. 8.

Comment

The proposed outfall design calls for a maximum flow of 5 mgd, which received no apparent BioWin simulation or treatment analysis

Response

The treatment facilities would never discharge 5 mgd and will not be designed to discharge more than 1.8 mgd. The outfall design maximum capacity is driven by standby/emergency disposal needs if there was a catastrophic failure to infrastructure that is critical to current disposal practices.

Comment

Total Phosphorus discharges would have to come down to 0.03 mg/L to meet all discharge requirements.

Response

False. Class A-1 effluent requirements do not regulate phosphorus at all. 0.03 mg/L of TP is the criterion for the Gallatin River, but only during the nutrient season (July 1st – September 30th), and dilution credits can be claimed to meet this criterion at the end of a mixing zone.

Comment

The AE2S study did not evaluate existing effluent concentrations relative to regulatory compliance. ..it does not compare regulatory effluent limits to collected values.

Response

Please read Technical Memorandum 14, and specifically see Table 2 “Screening Evaluation of Dilutions Required for New Submerged Outfall Based on Existing BSCWSD WWTF Effluent and Gallatin River Background Data”

Comment

The overall proposed schedule, treatment unit completion and basis for implementation will result in potential violations of water quality standards in the Gallatin River basin. The gap between treatment upgrades and the time allowed for river discharge or land application promotes extended periods of water, groundwater and soil quality violations.

Response

This is not true. The reviewer somehow misunderstood the timeline for MBR implementation (first project) and the results of the preliminary permitting evaluation, both of which reflect the District’s desire to achieve full compliance with water quality standards in the Gallatin River, groundwater and soil quality. The result of the MBR upgrade will be a Class A-1 effluent, which is the highest quality effluent considered in DEQ’s regulations.