Proceedings

Abstract

This article presents principles of Mixed Liquor Vacuum Degassing (MLVD) BIOGRADEX TM technology applied between final cells of bioreactors and secondary clarifiers of BNR WWTP’s. Utilisation of this process drastically changes characteristic of the activated flock structure and sludge settling characteristic by removal of gas micro-bubbles and reduction of dissolved Nitrogen gas concentration in liquid phase below saturation level, minimising activated sludge flock buoyancy with concurrent limitation of filamentous bacteria impact on secondary settlement process.

The MLVD process allows the plant to operate at almost double the conventional mixed liquor suspended solids (MLSS) concentration, with typical practiced concentrations of Z = 6,000 – 7,500 mg/L and the highest recorded MLSS concentration in the bioreactor of 12,000 mg/L. The use of MLVD process allows the plant operate at an average final clarifier solids loading rate SLR as high as 180-240 kg SS/m2d, with the highest recorded MLSS load exceeding SLR of 320 kg/m2d. The MLVD process allows the biological treatment process to be conducted with low biomass loading rate in the range of 0.05 kgBOD/kgMLSS, which results in very high reduction of total nitrogen and other effluent performance indicators (BOD, COD, TSS etc.).

The use of MLVD technology allows an easy control of MLSS concentration in the flow-through systems; the sedimentation process is conducted with a very high efficiency, eliminating activated sludge bulking in the clarifiers, floating solids due to denitrification and thus reducing solids carry-over at the clarifier’s effluent weirs. Application of the MLVD results in increase of flow and load capacity availability within existing plants infrastructure, minimising requirements for and costs of plant upgrades or expansion. The degassing technology saves space, increases plant throughput and allows for attainment of the increasingly more stringent effluent standards. Due to its high performance it may also limits requirements for tertiary treatment.

This paper presents case studies of existing wastewater treatment plants worldwide where MLVD technology have been utilised.

Keywords Activated sludge; degassing; total nitrogen removal; sludge bulking; MLSS control; high effluent standard; clarifier troubleshooting

Introduction Activated sludge mixed liquor suspended solids (MLSS) enter the secondary clarifiers from a well aerated and turbulent environment in the aeration basins. Sludge flocs contain micro-bubbles of gas, which make sludge settling difficult or even cause them to float. This buoyancy is further aggravated by hydrophobic foaming microorganisms such as Nocardia often present in biological nutrient removal (BNR) plants operating at long solids residence times SRT (Jenkins et al 2004, WEF 2006). The concentration of nitrogen gas dissolved in the water fraction of mixed liquor is at the saturation level as the result of oxygen consumption from the introduced air and simultaneous nitrification -denitrification. Because of this saturation nitrogen gas produced during denitrification processes occurring in the sludge blanket of the secondary clarifiers cannot dissolve in the surrounding water and forms gas bubbles which affect sludge settleability and create floating sludge or scum (Metcalf & Eddy, 2003). Operators of wastewater treatment plants optimize the plant performance balancing between sufficiently high MLSS concentration in the bioreactors required to achieve the design SRT and the ability of secondary clarifiers to effectively separate activated sludge from treated wastewater. Sludge bulking and foaming (scum formation) in bioreactors and secondary clarifiers are typically dealt with by reducing the MLSS through increased and often excessive, sludge wasting and chlorination of return activated sludge (RAS) to reduce the number of filamentous organism in sludge. Some plants strive to maintain a low sludge blanket in the secondary clarifier. As a result of these remedial measures plants may operate at MLSS concentrations lower than required to achieve the expected treatment results and have to be re-rated below its design capacity. Other plants may work well during dry weather flow conditions but encounter significant solids washouts from secondary clarifiers during wet weather flows.

Removal of gas bubbles from mixed liquor and reduction of dissolved nitrogen gas concentration below saturation level can reduce sludge settling problems related to these two factors. Activated sludge was found to readily separate from degasified mixed liquor and to settle well without formation of a layer of partially settled or floating solids. Denitrification processes occurring in settled sludge should not affect sludge settling since produced nitrogen gas dissolves in surrounding water instead of forming gas bubbles. The enhanced ability to settle and thicken in the final clarifier would lead to maintenance of normal solids surface load above 150 kg TSS/m2d. This in turn would allow for larger MLSS concentrations in the reactors leading to increased capacity of the plant without physical increase of the reactor size.

Mixed liquor degasification process has been developed over 20 years ago (Gólcz 2005) and has been applied in situations where plants have difficulties maintaining an effective year-round biological nutrient removal (BNR) performance. The process is also often used to expand capacity of the existing BNR plants, to convert carbonaceous plants to BNRs and to build new plants. There are some forty plants presently using this process (Maciejewski & Timpany 2007, 2008). This paper will present the principles of the degasification technology and its performance in full scale case studies conducted under a variety of conditions.