Role of Filamentous Bacteria and SRT

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CHAPTER II Combined Effect of Reactor Feeding Pattern and Cations on the Performance of an Activated Sludge SBR

ABSTRACT: Laboratory scale sequencing batch reactors (SBR) were used to study the effect of feeding pattern on activated sludge containing different concentrations of monovalent cations. Effluent wastewater quality and settling and dewatering properties were analyzed to determine if extending the feeding time to an SBR would have an impact on the effects of adding high amounts of sodium (Na⁺) to an activated sludge system, simulating wastewater treatment plants containing high quantities of monovalent cations in their influent. Data suggest that excess amounts of sodium caused poor flocculation in general, but when the feeding time was raised from 1 min (pulse feed) to 1hr and 4hr, an increase in deflocculation was observed. Deflocculation caused by the combined effect of sodium and feeding pattern was less when the sodium concentration was low. As sodium was increased to 6 meq/L, the negative effects on bioflocculation caused by the feeding strategy were offset. As the amount of sodium was further increased, a dramatic effect on settling and effluent water quality was observed reflected by an increment in effluent total suspended solids (TSS) and effluent chemical oxygen demand (COD) accompanied by an increase in sludge volume index (SVI) and capillary suction time (CST). An increase in effluent biopolymers (proteins and polysaccharides) was also observed. A better understanding of the effects produced by changing the feeding strategy on bioflocculation will help wastewater plant operators meet effluent requirements when struggling against high amounts of monovalent cations in their influent. Also, when performing analysis related to settling and dewatering in lab scale activated sludge reactors, the feeding pattern should always be reported in order to standardize procedures and compare results where the effects of changing the feeding strategies have been considered.
KEYWORDS: Activated sludge, cations, feeding pattern, sodium, bioflocculation, settling, biopolymers, sequencing batch reactor.

Introduction

The activated sludge process is widely used to treat both industrial and municipal wastewater. While most municipal treatment plants perform well, industrial facilities often struggle to meet effluent requirements. One reason for this is that the biomass from industrial facilities settles poorly. One reason for this poor performance has been attributed to a high monovalent to divalent ion ratio in many industrial wastewaters (Higgins and Novak, 1997).
The quality of the effluent from activated sludge treatment plants is highly dependent on the efficiency of the solid-liquid separation process. If this process is poor, wastewater treatment will be ineffective, resulting in failure to achieve regulatory effluent requirements. Solid-liquid separation is a physical process which involves the removal of solids particles that can be in a suspended, colloidal or soluble state. This is usually accomplished by gravity sedimentation. For a successful separation the microorganisms must clump together to form flocs of a defined size, porosity, density and strength to allow them to settle and compact well without leaving a high concentration of suspended solids in the effluent. Bacteria in suspended cultures, under the appropriate growth conditions, are able to grow and attach each other to form flocs. Flocs with very different properties and morphologies may occur, depending on the conditions in the activated sludge treatment plant and wastewater composition (Wilén et al., 2003).
Bioflocculation is one of the most important steps in building large, strong, dense, and compact settleable floc. For a better understanding on microbial flocculation, the structure and components of the floc must be defined in order to propose means to change their properties and improve the settling and dewatering properties of the activated sludge. Activated sludge floc consists primarily of biopolymer, cations, microorganisms (floc forming bacteria and filamentous bacteria), and debris trapped within the floc. These biopolymers referred to as exocellular polymeric substances (EPS), are produced when the active biomass converts complex organic matter into low molecular weight compounds (Sponza, 2004), forming a matrix that encapsulates the microbes and aids in the aggregation of the microorganisms.
The production of EPS, which is mainly composed of carbohydrates, proteins, nucleic acids, lipids and humic substances, is believed to be dependent of the growing phase, decay and lysis of bacteria. Researchers suggest that EPS, both in terms of quantity and quality, are very important for the floc properties of the activated sludge (Liao et al., 2000). A combination of floc-forming bacteria and filamentous bacteria compose the macrostructure of flocs, with some of the filamentous bacteria enmeshed inside the floc to provide a solid structure. The relative abundance of filamentous organisms will depend on several factors such as solids retention time and substrate concentration. Several studies also suggest that feeding patterns also have a strong influence on the microbial community of activated sludge (Martins et al., 2003).
Cations significantly affect bioflocculation and alter the settling and dewatering characteristics of activated sludge systems. Cations imbalances are a common cause of sludge settling problems especially in activated sludge plants related to industrial activities (Higgins and Novak, 1997a). High concentrations of monovalent cations such as sodium (Na⁺) are detrimental to activated sludge properties (Park et al., 2006). It has been demonstrated that high concentrations of sodium present in activated sludge result in a deterioration of sludge properties such as, capillary suction time (CST), sludge volume index (SVI), effluent TSS and effluent COD. Sodium in the influent wastewater also causes an increase in proteins and polysaccharides in the effluent (Murthy and Novak, 2001). Researchers suggested that at values lower than 10 meq/L sodium did not impact greatly the settling properties of the activated sludge, but at concentrations higher than 10 meq/L, poor settling will occur (Higgins and Novak, 1997c).
It has been proposed that the divalent cations act as a bridge between the negatively charged particles within the biopolymer network but in the presence of monovalent cations they are displaced by an ion exchange process that reduces the ability of biopolymer to bind and form a good floc matrix (Higgins and Novak, 1997c). Higgins and Novak (1997b) suggested that a relationship between the sum of monovalent and divalent cations (M:D) could be a good indicator to evaluate the problems related to settling and dewatering; they suggested that with a ratio greater than 2 there would be considerable problems in effluent quality associated to settling (Higgins and Novak, 1997a).
The above-mentioned studies describe many of the mechanisms involved in bioflocculation, which consider several aspects that will influence the floc formation process, but these are not the only factors that will affect effluent water quality, settling and dewatering properties of activated sludge. While performing lab-scale studies related to cations in sequencing batch reactors (SBR), Higgins and Novak (personal communication) observed that the way in which the SBR was fed influenced the results obtained for analysis related to effluent quality and stated that differences in feeding pattern will probably have a dramatic effect on the sludge setting properties in SBRs. They observed that by increasing the length of time of the feed, which creates a low substrate concentration in the reactor, affected negatively the settling properties of the activated sludge. However, when the fill time was short, a high substrate gradient was present which results in the substrate intake to be close to the maximum specific rate of bacteria and appears to lead to good sludge settleability. These studies supported the idea that the feeding pattern had an influence on the microbial population dynamics and kinetics of activated sludge (Martins et al., 2003). The competition and selection of microorganism inside a reactor will be associated with properties such as growth rate, substrate intake rate, and substrate affinity, which will then affect the quality and quantity of EPS produced.
The objective of this study was to determine if the effluent quality, settling and dewatering properties of lab-scale SBR reactors at different sodium concentrations were to experience any change when subject to different feeding patterns, and to determine which feeding configuration yields the best results regarding effluent quality. If the deterioration was eminent, then considerations in controlling influent flows should be taken when operating industrial or municipal plants where the M:D ratio might be high due to the usage of sodium based chemicals for pH control, and to encourage other researchers to include the impact of feeding pattern when performing studies related to solid liquid separation in activates sludge systems.

Materials and Methods

Experimental Approach. Experiments were conducted using four sequencing batch reactors (SBR), each one containing a 1 L volume. Since the purpose of this study was to analyze the effect of the feeding pattern in the effluent quality and sludge settling properties when exposed to different sodium concentrations, different feed configurations were required. A phase, as denoted, represents the period of time when four reactors containing different amounts of sodium are operated under the same flow conditions. Each phase has a unique feeding pattern which is maintained constant through the entire operational period. Each phase is characterized by different filling time duration, as shown in table 2-1. A pulse feed was used for phase one (feeding time: one minute), for phase two, one hour, and for phase three, a four hour feed time was used.
The reactors were seeded with mixed liquor obtained from the Blacksburg, VA., municipal wastewater treatment plant. The SBR were operated with two cycles per day, each cycle lasting for 12 hours. The reactors were operated using a 1 day hydraulic retention time (HRT) and an 18 day solids retention time (SRT). This SRT was chosen to maintain a mixed liquor suspended solid concentration of around 2,000 mg/L to allow nitrification and therefore reduce the effects of the ammonium (NH₄⁺) ion, and to reduce the effect of SRT in floc formation.
Oxygen was provided using compressed air fed through diffuser stones to allow a dissolved oxygen concentration greater than 2.0 mg/L and to provide enough mixing to keep the biomass in suspension without disturbing the process of floc formation. The oxygen input was regulated in a way that it did not favor the growth of filamentous organisms. The pH was monitored but was not controlled (one of the added salts contained HCO₃ which added some buffer capacity). The temperature for the entire study was maintained at 20º C.
Feed and Cations. Bactopeptone, a microbiological enzymatic digest of protein for use in culture media, was used as feed (electron donor, carbon, and nutrient source) for the reactors at a concentration of 300 mg/l as COD. Several COD (chemical oxygen demand) analysis were done to determine the required bactopeptone concentration that would give the desired value. It was determined from this analysis that 273 mg/L of bactopeptone provided a COD of 300 mg/L. The low concentration of cations in the bactopeptone allowed control of cations to the feed as described in table 2-2.
Table 2-2 shows the cations that were added to the feed. The concentrations of all the cations were maintained constant during the three phases of the experiment. The inorganic salt concentrations in the feed (excluding sodium) were chosen in a way that they had a minimal impact on the settling and effluent properties of activated sludge following the recommended values found in the literature (Higgins and Novak, 1997c, Murthy and Novak, 1998, Park et al., 2006) .

CHAPTER I Literature Review 
1.1 Background information on wastewater treatment
1.2 Solid-Liquid separation
1.3 Bioflocculation
1.4 Role of Filamentous Bacteria and SRT
1.5 Cations and there effect in settling
1.6 Sequencing Batch Reactors (SBR) and Feeding Pattern
CHAPTER II Combined Effect of Reactor Feeding Pattern and Cations on the Performance of an Activated Sludge SBR
Introduction
Materials and Methods
Results and Discussion
Implications
Engineering and Scientific Significance.
Conclusions
References 
Appendix .
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EFFECT OF REACTOR FEEDING PATTERN ON PERFORMANCE OF AN ACTIVATED SLUDGE SBR