The reliability and accuracy of in-situ ion selective electrode and ultraviolet (NO) probes have been investigated at four different treatment plants with different operational conditions. This study shows that the mentioned probes tend to compromise their accuracy and trending stability at lower NO of <1.0 mg N/L, which if used as a measuring variable for PI feedback controller for denitrification (biological reduction of nitrate to nitrogen gas), would cause overfeeding the external carbon source. In-situ Clark-type NO sensors, recently introduced for industrial scale use (Unisense Environment) could potentially open a new horizon in the automation of biological processes and particularly denitrification. To demonstrate the applicability of such probes for automation, two in-situ NO probes were used in two treatment plants in parallel with NO-N probes. The effects of operational conditions such as COD/N ratios and the correlation between NO and NO were investigated at those plants. NO production at non-detect dissolved oxygen concentrations and pH of 7-7.2 were found to be a function of influent nitrogen load or the ratio of COD/N. Finally, using an NO probe as a proxy sensor for nitrates is proposed as a measured variable in the PI feedback in the automation of the denitrification process with a NO set point of <1.2 mg N/L).

This paper presents the upgrade of the Upper Blackstone Water Pollution AbatementDistrict's (UBWPADs) sludge incinerators to meet the requirements of an Administrative Consent Order (ACO) and to improve the performance of the incinerators. Initially UBWPAD had three (3) MHFs installed in the mid 1970's. The incinerators were upgraded in 1996 by the inclusion of the Re-Heat Oxidation (RHOX) process (i.e. the addition of Electrodynamic Venturis (EDVs) and regenerative thermal oxidizers (RTO's) to improve control of volatile organic compounds (VOCs) and carbon monoxide (CO).Following this upgrade there were numerous bypass events (i.e. release of uncontrolled emissions to the atmosphere through the furnace's emergency dump stack). The uncontrolled emissions caused regulatory problems for the District. In addition, the District had problems with plugging of the RTOs due to the poor performance of the EDV systems in removing particulate matter. In the past, the RTOs would become so plugged with particulate matter after about 3 to 4 months of operation that the unit would have to be taken off line and water washed. Lastly, the District had continual problems with slagging and clinker formation in the furnaces which caused unexpected furnace outages and which significantly increased the manpower requirements to operate and maintain the furnaces. Due to the slagging and clinker problems, the District had voluntarily reduced the furnace throughput to not more than 1.3 dry tons per hr (DTPH), even though each unit had a rated capacity of 2.3 DTPH, based on a 23% solids feed.To address the above operational problems, the District decided to upgrade the MHFs. The upgrade essentially consisted of the installation of new wet electrostatic precipitators (WESPs) and flue gas recirculation (FGR) systems on the multiple hearth furnaces (MHFs). The WESPs were installed to improve the particulate removal of the incinerators' existing air pollution control (APC) systems. The FGR systems were installed primarily to reduce the slag and clinker formation in the furnaces which were causing excessive furnace downtime and requiring significant manpower for slag and clinker removal.outlet of the WESPs demonstrated that the new wet scrubbing and WESP systems achieved average outlet grain loadings of 0.000755 grains per dry standard cubic foot corrected to 7% oxygen (gr/dscf 7% ) and 0.000416 gr/dscf 7% on Incinerators 1 and 2, respectively. These particulate emissions were well below the permit limit of 0.014 gr/dscf.After four months of operation the operating RTO was inspected and there was no build up of particulate matter and no need to water wash or clean the media. Thus the new APC systems were accomplishing their design objective which was to prevent plugging of the RTOs. At the 4 month interval, the operating MHF was also inspected and there was essentially no clinker or slag formation and no need to deslag the furnace. Hence the FGR system was accomplishing its intended purpose which was to prevent clinker and slag formation in the ...

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