5 years ago

Preliminary Engineering Report - Fauquier County

Preliminary Engineering Report - Fauquier County

without nitrogen control

without nitrogen control facilities). As such, DEQ is likely to impose a more stringent effluent limit on (total) nitrogen than what the existing pretreatment system is capable of producing, and therefore further nitrogen removal will be required in the post-treatment stage, for both Level 2 and Level 1 effluent quality. Since the majority component in the pretreated leachate is nitrate-nitrogen (NO3-N) a high degree of denitrification will be required in the post-treatment system. Therefore, a biological posttreatment system is recommended providing for both denitrification as well as oxidation/nitrification for remaining ammonia and degradable organics, the latter being important during periods where the pretreatment system may be operating less efficient. The post-treatment process will be designed with much higher loading rates compared to the current pretreatment pond and thus the system will have a small and compact footprint. The system will include a staged biological reactor (bio-reactor) with a solids removal/filtration step which can be standard filtration or an ultra-filtration membrane. The bioreactor can be of the activated sludge type, either as a continuous flow reactor or a sequential batch reactor (SBR), or it can be of the fixed-film/attached growth type. A biological activated sludge process with an ultrafiltration (UF) membrane for solids separation, typically referred to as Membrane Bio-Reactor (MBR), is a high rate process with a relatively small footprint as depicted below (“DynaLift”® from Dynatec Systems). The membranes are skidmounted outside the liquid treatment reactors. The package system is complete with reactor tanks, pumps, air blowers/piping, membrane cleaning system and control panels. Small-scale MBRs are widely used for decentralized treatment and also used at a number of facilities treating landfill leachate; one example is a 35,000 gallons per day facility in Warren County, NJ. The current pretreatment process in Pond #2 (AquaMats Biofiltration®) is an attached growth system (as opposed to an activated sludge suspended growth system) which is simple to operate and has worked well for Corral Farm landfill. The Moving Bed Bio-Reactor (MBBR) process is an attached growth system with small plastic elements suspended (aerated/mixed) in the reactor on which the biofilm can attach and provide treatment (illustrated below). The plastic elements (dime to quarter size biofilm carriers) are retained in the reactor by stainless steel screens with approximately 10 mm mesh size while allowing solids to flow through. A solids separation step is provided for the MBBR effluent, via a clarifier, filter or membrane. In this case, an ultra-filtration (UF) membrane is suggested similar to the MBR discussed above to achieve a consistent and reliable effluent for reuse. N:\18469-000\Engineering\Reports\April 2012 PER\18469 PER Treatment System 7-31-2012_AM.docx Page 13

Both systems, the activated sludge/MBR process and the attached growth MBBR-UF Membrane process, are expected to provide similar level of treatment. A key difference is that the MBBR is a straight flowthrough process with no return (activated) sludge and thus has a much lower solids load on the membrane compared to the activated sludge process. However, for the purpose of this Preliminary Engineering Report it is assumed that a package activated sludge/MBR system will be provided. A final selection of the type of treatment system will be made during the preliminary design stage. Since the organics (BOD)-to-nitrogen ratio is very low in the pretreated leachate, an external carbon source is needed for the denitrification. This is assumed to be in the form of methanol, but could also include an alternative source such as acidic acid, sugar-based waste products, or high quality waste glycerin. The external carbon source will be added in the first (“anoxic”) stage of the bio-reactor as well as the last (“anoxic”) stage. The estimated daily carbon usage (methanol) is less than 5 gallons. It is also recommended that a 2 mm fine screen be provided upstream of the treatment process to provide additional protection for the membranes. Bacterial disinfection of the treated effluent is required and can be provided by chlorine addition (sodium hypochlorite) or ultra-violet (UV) lighting. An in-line / closed pipe UVdisinfection system (depicted below, (“Wedeco”®)) is recommended to eliminate the need for chemical handling and process control equipment associated with chlorine addition. It should be noted that high total dissolved solids (TDS) and some coloring, often associated with landfill leachate and not usually removed in the treatment process (even with UF-membranes), can potentially impact the effectiveness of UV-disinfection (due to reduced transmittance of UV light). As such, it is recommended that pretreated leachate samples be collected during the design phase and tested by a UV manufacturer. N:\18469-000\Engineering\Reports\April 2012 PER\18469 PER Treatment System 7-31-2012_AM.docx Page 14

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