Effect of solids retention time on <i>Microthrix parvicella</i> growth

Abstract

The objective of this study was to evaluate the effect of solids retention time (SRT) on M. parvicella growth and to calculate growth kinetic parameters of this filamentous species. Bench-scale continuous-flow experiments showed that M. parvicella growth can be significantly suppressed at an SRT of lower than 5.7 d for temperatures of between 14 and 18 o C. According to the continuous-flow experiments the maximum sludge age for the avoidance of filamentous foaming problems caused by M. parvicella is 6 d for temperatures lower than 18 o C. At this sludge age M. parvicella loses its hydrophobicity and therefore its foaming potential. However, even lower SRTs are required in order to achieve a significant suppression of its growth. At SRT values of less than 5.7 d M. parvicella initially forms a shorter filament (< 150 μm) with clear spaces inside filaments and variable Gram stain reaction and eventually is eliminated from activated sludge biocoenosis. According to kinetic studies presented in this paper, M. parvicella is a slow growing bacterium with a low maximum specific growth rate of 0.67 1/d and 0.53 1/d under aerobic and anoxic conditions respectively. Maintenance energy requirements of M. parvicella were found to be significantly lower than the maintenance energy of floc forming micro-organisms as well as other filamentous species, thus providing the micro-organism with a significant advantage under starvation conditions prevailing at the majority of the extended aeration activated sludge systems.

Causes and remedies for filamentous foaming in activated sludge treatment plant

“…decreasing mean cell residence time) (Noutsopoulos et al, 2006)  additional structures (e.g. classifying selectors) (Chudoba et al, 1973;Caravelli et al, 2003)  controlling dissolved oxygen levels in the pre-oxidation reactor (Pasinetti et al, 2005)  non-specific measures such as water sprays, steam application (Noutsopoulos et al, 2006).…”

“…Although the foaming bacteria grow faster on fatty, oily substances, they still grow relatively slowly as the fats, greases and oils are difficult to digest (Griffiths, 2010). It is suggested that the percentage of excessive bulking incidents is 10% to 29% for a SRT of greater than 8.5 d and filamentous bacteria requires a minimum sludge age of 10 days to grow and form a stable population (Griffiths, 2010) whereas there is no bulking occurs for SRTs of lower than 7 d (Noutsopoulos et al, 2006). Most early ASPs were focusing only on the biochemical oxygen demand (removal of organics) and operated at relatively short sludge ages of maximum 3 to 4 days.…”

“…Specific control strategies are preferable as they are selective and offer a permanent solution of the problem whereas non-specific methods tend to provide only temporary solutions. Noutsopoulos et al (2006), based on laboratory scale studies has shown that reduction of sludge age may prove to be an efficient method to suppress M. parvicella growth. Sludge age reduction method cannot be applied in activated sludge systems that need to nitrify throughout the year, because the sludge age required to washout M. parvicella, may also eliminate nitrifying bacteria.…”

Filament identification and dominance of Eikelboom Type 0092 in activated sludge from wastewater treatment facilities in Cape Town, South Africa

“…Indeed, M. parvicella, Type 0092 and Type 0041/0675 are grouped together (Group IV) as aerobic-anoxic-anaerobic-zone growers which are present in conditions of high solids retention, possibly utilising particulate substrates for growth (Martins et al, 2005). Although BNR WWTWs with low F/M ratios, high solids retention times and predominantly particulate and slowly biodegradable COD (SBCOD) have been associated with the selection of Type 0092 (Eikelboom, 2000;Casey et al, 1999aCasey et al, , 1999bCasey et al, , 1999cJenkins et al, 2004;Martins et al, 2003), there is also research which discounts the influence of F/M ratio on the selection process (Noutsopoulos et al, 2006).…”

“…High concentrations of fatty acids are formed when there are long retention times in sewers, primary settling tanks or anaerobic zones, which provide ideal conditions for the proliferation of M. parvicella (Jenkins et al, 2004). Under laboratory conditions, it has been shown that foaming problems due to M. parvicella at low temperatures can be alleviated by reducing the age of the sludge to change the morphology of the filament (Noutsopoulos et al, 2006), and that that the presence of free ammonia as a source of nitrogen can cause bulking by M. parvicella (Tsai et al, 2003). Specific strategies that either have been used (with varying degrees of success) or show promise for the control of filamentous overgrowth of M. parvicella include: reducing solids retention times, using pre-flotation to reduce the lipid fraction, increasing the dissolved oxygen concentration (ideally by employing selectors), and adding polyaluminium hydrochloride (Jenkins et al, 2004;Madoni and Davoli, 2002;Nielson et al, 2002;Roels et al, 2002).…”