Factors affecting the reliability of results using 1,10‐phenanthroline (phen) in the quantitative assay of minerals for Fe2+ and total Fe were studied. The greatest source of variability was the photochemical reduction of ferric‐phen species during Fe2+ analyses. It is believed that oxo‐bridged dinuclear complexes of Fe2+ and phen are formed in the mineral digests, which then are susceptible to photochemical reduction by wavelengths < 400–500 nm. The rate of photoreduction of a solution obtained by digesting oxidized nontronite was sufficient to increase the predicted Fe2+ content of the clay from about 0 to 2.45% by simply leaving the sample for 1 hour in normal fluorescent light in the laboratory; and for partially reduced nontronite, from 3.25 to 4.68%. The rates of increase for the two clays were 0.0024 and 0.0014 absorbance units/min, respectively. Interferences due to this phenomenon were eliminated when samples were kept in the dark or under subdued red light. Other factors that adversely influence the accuracy and precision of the standard curve included (i) the amount of 48% HF used to digest the sample; (ii) addition of chemical reducing agents such as hydroxylamine hydrochloride and hydroquinone; (iii) the order in which phen is added relative to other reagents; (iv) pH; and (v) the length of time allowed for color development if phen is added only to the final dilution.
The Metro Wastewater Reclamation District (Metro District) provides wholesale wastewater transmission and treatment service to 55 local governments in the Denver metropolitan area. Solids processing at the Central Treatment Plant (CTP) consists of dissolved air flotation (DAF) thickening of waste activated sludge (WAS), anaerobic digestion of blended primary sludge and thickened WAS, dewatering with high-solids centrifuges, land application at the Metro District's METROGRO Farm located approximately 100 miles east of Denver, and composting of a portion of the digested biosolids. Prior to this project, the Metro District had 10 anaerobic digesters operated in parallel in a high-rate mesophilic digestion process. Based on flow and load projections, a capacity expansion of the process was necessary to meet the needs of the Metro District.CH2M HILL was retained by the Metro District to evaluate ways to meet its future sludge digestion needs and provide additional digestion capacity through the design year 2020. Following evaluation of numerous alternatives, the decision was made to construct two additional digesters (Digesters 11 and 12) to provide the additional needed capacity. In addition, the decision was made to provide the flexibility to operate the digestion process in either a single-stage mode or a two-phase (acid/gas) mode. Key components of the construction project included:• Two new anaerobic digesters, designed to operate as either acid, methane, or combined modes as part of either a single-stage or two-phase process. • A new Digester Control Building, improvements to the existing Digester Mall, and improvements to the existing anaerobic digesters. • Improvements to the hot water system. Hot water is created by waste heat from the CTP's Cogeneration system, with three natural gas fired boilers serving as back up. • Improvements to the digester gas handling system, including four new waste gas flares.• Acid sludge pumps and digested sludge pumps.• Process Control and instrumentation upgrades.The innovative two-phase process separates the two biological steps of anaerobic digestion into separate vessels for maximum efficiency. The two-phase system consists of highly loaded "acid phase" digesters followed by lightly loaded "gas phase" or "methane" digesters. The first stage provides hydrolysis and acidification pretreatment, and the second stage maximizes gas production. The two-phase process is anticipated to result in improved volatile solids reduction, higher gas production, and improved process stability. The new digesters were placed into operation in late 2005 and the digestion process was converted to two-phase in February 2006. 6752 WEFTEC®.07This paper will briefly summarize the main components of the project and explain the two-phase digestion process and how it is operated. It will focus primarily on describing the lessons learned in starting up the new facilities and the process optimization efforts completed after startup. It will relate the impacts of a separate DAF thickening and primary slud...
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