EFFECTS OF VARIOUS CARBON SOURCES ON THE PERFORMANCE AND PROPERTIES OF DENITRIFIYING SLUDGE

Biological denitrification is one of the most cost effective and widely used methods for nitrogen removal in municipal wastewater treatment plants. Under new stricter South East Queensland effluent guidelines the performance of wastewater treatment plants has to improve significantly to meet effluent total N restrictions of 5 mg N/L. The organic carbon contained in municipal wastewater that was utilised to carry out denitrification is often not sufficient to meet these stricter guidelines. As such the addition of an external carbon source is becoming a viable option to increase the COD (chemical oxygen demand) supply to a treatment plant to enhance denitrification. The choice of carbon source has long been a question of cost and performance. Often methanol has been the preferred choice of operators because of its relatively low cost per carbon mole, and the stability it provides with long term use. However other carbon sources are attractive in some cases because of their comparatively low cost, their ability to enhance denitrification. Such carbon sources include, among others, molasses and sludge hydrolysate. This thesis investigated the long-term effects of methanol, molasses and sludge hydrolysate on the performance and properties of denitrifying sludge when used as external carbon sources. Studies were carried out on both full-scale wastewater treatment plants and lab-scale reactor systems fed with real wastewater. The denitrification performance as well as a number of key properties of the sludges receiving an external carbon source were monitored and compared to those of a reference system. An effort was made to design fluorescence in-situ hybridisation (FISH) probes using the stable isotope probing (SIP) method for the measurement of the entire denitrifying community. In addition, the impact of various storage conditions on the maintenance of denitrification activities was investigated. The key outcomes are summarised below. Ethanol as an alternative carbon source for methanol adapted denitrifiers At the South Caboolture Water Reclamation Plant a 2 sludge post-denitrification reactor is used to polish the effluent before discharge, with methanol used as the primary external carbon source. The capability of the bio-films to denitrify with methanol, ethanol and acetate as sole or dual carbon sources was monitored for a period of one year using batch tests. On average the highest rates of denitrification were achieved using methanol, however ethanol supported denitrification rates were very similar to those with methanol (&gt90%). Acetate supported denitrification varied considerably during the study period being as low as 15% of the average rate achieved with methanol, however after 2 weeks of ethanol addition the acetate supported denitrification rate had increased to be identical to the rate achieved with ethanol. The experimental results suggest that three functional bacterial groups were present within the microbial community of the biofilms, with the dominating group being able to use both methanol and ethanol as carbon sources for denitrification. The results of this study indicate that the use of ethanol as a short-term replacement for methanol in times of methanol unavailability is a sound operating option. Long term addition of molasses and fermented molasses: Impact on nitrogen removal and activated sludge properties The long term addition of molasses and fermented molasses were investigated for their impact on nitrogen removal and other operating parameters. The study was conducted in a bench scale SBR reactor operating as single sludge pre-denitrification system in parallel with a control reactor. The results of the study showed that fermented molasses was the better carbon source for enhancing nitrogen removal, removing 8.69 mg N/L more than the control reactor receiving no external carbon at a COD addition of 100 mg TCOD per litre wastewater added. The long term addition of fermented molasses led to periods of bad settling however resulting in high SVI values throughout the addition period which appeared to be the result of the enumeration of filamentous organisms. Biomass washout was a feature of the long term addition of both fermented and non fermented molasses as a result of the poor settling. Long term addition of molasses/fermented molasses resulted in adaptation of the biomass, resulting in the biomass having an increased ability to denitrify with acetate, butyrate and wastewater COD. The use of either fermented and non-fermented molasses caused problems with the sludge settling characteristics of activated sludge in the lab scale SBR reactor. Long term addition of sludge hydrolysate: Impact on nitrogen removal and activated sludge properties The long term effect of the addition of sludge hydrolysate on the nitrogen removal and other operating parameters was investigated in this study. The study was conducted in a bench scale SBR reactor operating as a single sludge pre-denitrification system in parallel with a control reactor. The liquid from a rotary sludge thickener (RST) at a full-scale wastewater treatment plant was used as the hydrolysate. The RST liquid was found to contain a large fraction of volatile fatty acids, with acetate being the dominant species. The long term addition of the sludge hydrolysate was found to enhance biological denitrification with a removal rate of 3.64 mg N/L with a hydrolysate addition of 40 mg COD per litre wastewater added. During the long term addition of the hydrolysate a decrease in the nitrification performance led to an accumulation of ammonium up to 30 mg NH4-N/L. It was unclear if this was a result of the hydrolysate addition. A period of 2 weeks was required for the nitrification performance to return to normal and the excess ammonium removed. The formation of pin flocs with poor settling was also a feature of long term hydrolysate addition which is potentially related to the addition of acetate in the hydrolysate. It would appear that high VFA containing sludge hydrolysates would make a better short term carbon source, in preference to long term addition. Impact of aerobic, anoxic, intermittently aerated and anaerobic storage conditions on denitrification activity of activated sludge The storage of excess sludge has been shown to be a reliable method for reducing the size of treatment plants. Under storage however the activity of bacteria can deteriorate. The rate at which this deterioration occurs is an important factor in determining the method of storage applied. This study investigated the impact of four storage conditions (aerobic, anoxic, intermittent aeration and anaerobic) on the activity of denitrifiers. The aerobic storage conditions yielded the highest VSS decay rate of 0.028 d-1, followed by the intermittent aeration (0.020 d-1), anoxic (0.012 d-1) and anaerobic (0.009 d-1) storage conditions. Storage under intermittently aerobic conditions resulted in the lowest denitrification rates measured in the study, with a maximum nitrate reduction rate of 4.97 mg NO3-N/gVSS/h compared to the seed sludge nitrate reduction rate of 9.62 mg NO3- N/gVSS/h. The storage of sludge under anoxic conditions yielded the most stable denitrification rates with a nitrate reduction rate almost identical to the rates measured on day 1 (6.39 mg NO3-N/gVSS/h) even after 21 days (6.97 mg NO3-N/gVSS/h). For each storage condition tested an unusual trend over the first 5 days was measured with the denitrification rates all increasing. After day 5 the maximum denitrification rates decreased with the rate of decrease varying dependant on the storage conditions. Modified 15N-SIP method to investigate natural denitrifiers in activated sludge using carbon sources contained in wastewater 15N-SIP is a SIP method which can be applied to complex microbial systems which cannot be targeted using a carbon based SIP approach. A methodology was developed for this study, which involved (1) identifying a method that could remove the ammonium contained in raw wastewater, while retaining a significant amount of the natural soluble COD to provide an 14N-ammonium free wastewater feed source for the SIP incubation, (2) the incubation of a fresh sample of activated sludge from a full scale wastewater treatment plant with 15NH4 15NO3 to ensure only 15N was supplied to actively growing bacteria under anoxic conditions, and (3) to carry out RNA SIP because it is considered a more responsive biomarker that labels active bacteria more efficiently than DNA SIP. The development of an ion-exchange column for treating the wastewater to remove the 14N-Ammonium while retaining 50% of the volatile fatty acids to provide a natural carbon feed source for the SIP incubation was achieved. The SIP incubation appeared to be successful based on the cyclic studies conducted over the course of the incubation. However, the complete analysis of the RNA samples collected could not be achieved because the labelling efficiency of the incubation could not be confirmed and the experiment was deemed unsuccessful. While the overall experiment was a failure it is believed that it would be possible to successfully apply this method in the future with the knowledge gained