Development of aerobic granular sludge technology for domestic wastewater treatment in hot climates

Abstract

Conventional domestic wastewater treatment plants based on activated sludge technology require large footprint (big settling tank) due to the relatively slow settling characteristics of sludge flocs. Aerobic Granular Sludge (AGS) technology offers a possibility to design a compact system based on simultaneous organic and nutrient removal and because of the good settling characteristics of the AGS, the use of a big settling tank is not necessary. Therefore, the installation can be more compact, at a cheaper cost. The aim of this study was to develop AGS system for domestic wastewater treatment applications in hot climate conditions. Special emphasis is given to the settling characteristics and physical strength of the AGS. Therefore, a 3-litre laboratory-scale reactor known as Cyclic Aerobic Granular Sludge Bioreactor (CAgSBio) was designed and used. The operation of the reactor was based on the Sequencing Batch Reactor (SBR) system with a complete cycle operation of three (3) hours and specifically designed to be operated for twenty-four (24) hours continuously at temperature of 30oC. A 3-litre laboratory-scale reactor in Delft University of Technology (TU Delft), the Netherlands operated at 20oC was also used to compare the results on AGS granulation and performance. AGS developed at this low temperature and from a 1.4 m3-pilot plant at Ede Wastewater Treatment Plant, the Netherlands (fed with pre-treated domestic wastewater) were also used for a physical strength study, to compare with AGS at 30oC. All analytical measurements performed in this study were conducted according to Standard Methods for the Examination of Water and Wastewater (APHA, 2005). The study shows that after ninety (90) days of operation, stable AGS (fed with synthetic wastewater) with average size of 1.1 mm were formed at 30oC. To demonstrate the simultaneous organic and nutrient removal by AGS, a removal study was also conducted. CAgSBio system showed stable removal performance. Average removal efficiencies during steady state cycles at 30oC of organic carbon, total inorganic nitrogen and phosphorus reached 100%, 94% and almost 98% respectively. A study with actual wastewater (pre-treated domestic wastewater) at 30oC indicates that the granulation process does occur but at a slower rate (125 days is essential to develop mature granules) compared to synthetic influent (90 days). Meanwhile, AGS settling behaviour study shows that AGS settled relatively fast (velocities >12 mh-1 for size > 0.2 mm) compared to other conventional sludge flocs. The study also indicates that excessive mixing is not favourable for AGS reactors. Thus, the mixing criteria for AGS reactors was developed based on results obtained through experiments under mechanical and aerated-mixing conditions. Finally, a procedure was developed to evaluate the AGS strength based on the stability of AGS against shear stress. Determination of a stability coefficient (S) was introduced as an indicator of AGS strength. The results shows that AGS at 20oC and 30oC, fed with synthetic wastewater are very stable. For AGS fed with pre-treated domestic wastewater, the AGS at 30oC (produced using the laboratory-scale reactor) is more stable than the AGS at 20oC (produced using the pilot plant). As a conclusion, stable and compact AGS can be developed and performed effectively in hot climate conditions for domestic wastewater treatment applications, particularly as an alternative technology which is compact, high speed operation process (3 hours complete cycle) and more efficient.