SRT contributes significantly to sludge reduction in the OSA-based activated sludge process

Reduction of sewage sludge and N2O emissions by an Oxic Settling Anaerobic (OSA) process: The case study of Corleone (Italy) wastewater treatment plant

Biosolid management is becoming one of the most crucial issues for wastewater treatment plant (WWTP) operators. The application of the Oxic Settling Anaerobic (OSA) process allows the minimisation of excess sludge production. This study compares conventional activated sludge (CAS) and OSA layouts in a full-scale WWTP (namely, Corleone - Italy). Extensive monitoring campaigns were conducted to assess treatment performances regarding carbon and nutrient removal, greenhouse gas (GHG) emissions, excess sludge production, and biomass activity (by means of respirometric analysis). Results showed that the effluent quality consistently met the Italian discharge limits. However, with the implementation of the OSA process, there was a decrease in ammonium removal efficiency, which could be attributed to reduced nitrifier activity related to reduced biomass production and extended anaerobic conditions affecting the nitrification process. On the other hand, the OSA configuration significantly increased phosphorus removal, indicating a high phosphorus content in the resulting waste sludge. A worsening of the sludge settling properties was observed with the OSA configuration likely due to decreased EPS concentrations. The sludge production in the OSA configuration decreased by 17.3 % compared to CAS. Nitrous-oxide measurements did not show a variation between CAS and OSA configurations, confirming that the OSA process can be a suitable solution for reducing WWTP's carbon footprint.

Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge - oxic settling anaerobic system

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRTSHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < -91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (TSHT) from ambient (19.8-25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Yobs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Yobs decreased by 32% and 46-65% at HRTSHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Yobs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRTSHT of 6 h and when TSHT was set at 35 °C. OSA implementation did not affect COD and NH4+ oxidation of the IFAS system (90-96% and 99%, respectively) and improved total nitrogen (TN) reduction (31-53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.

Sludge minimization in mainstream wastewater treatment: Mechanisms, strategies, technologies, and current development

Excess sludge production in wastewater treatment plants has become an enormous environmental issue worldwide mainly due to the increased efforts towards wastewater purification. Researchers and plant operators are looking for technological solutions to reduce sludge production through the upgrading of existing technologies and configurations or by substituting them with alternative solutions. Several strategies have been identified to reduce sludge production, including the use of biological and physical-chemical methods (or a combination of them) and novel technologies, although many have not been sufficiently tested at full-scale. To select the most suitable system for sludge reduction, understanding the reduction mechanisms, advantages, disadvantages, and the economic and environmental impact of each technology is essential. This work offers a comprehensive and critical overview of mainstream sludge reduction technologies and underlying mechanisms from laboratory to full scale, and describes potential application, configuration, and integration with conventional systems. Research needs are highlighted, and a techno-economic-environmental comparison of the existing technologies is also proposed.

Simultaneous sludge minimization, pollutant and nitrogen removal using integrated MBBR configuration for tannery wastewater treatment

An advanced operational configuration of anoxic-aerobic moving bed biofilm reactors (AMOMOX process) was experimentally demonstrated to achieve simultaneous sludge yield minimization, pollution and nitrogen removal. The AMOMOX experimentation witnessed considerable variation in process parameters while feed operation changed from synthetic wastewater to real tannery influent. The strict maintenance of operational strategies resulted prominent removal of TCOD, SCOD, ammonia nitrogen and total nitrogen higher upto 93.5%, 94.8%, 95.2% and 88.7% respectively. The nourishment of filamentous microbiota and purposeful promotion of cell-lysis effectively sustained sludge yield restriction. Here, the sludge yield (Y_obs) lowering upto 0.51 gVSS/gCOD ultimately turned an overall sludge minimization of 46.8% compared with a parallel-run conventional activated sludge treatment. The observations were further supported by sophisticated instrumental imaging, thermogravimetric analysis and batch digestion test of the sludge pool. The experimental Y_obs and corresponding solids retention showed consensus with the reported correlation model and, thus, a modified correlation was tested.

Evaluation of sludge reduction in an oxic-settling-anoxic system operated with step feeding regime for nutrient removal and fed with real domestic wastewater

In the present study, the effect of sludge age was evaluated for simultaneous sludge reduction and nitrogen removal in an oxic-settling-anoxic (OSA) system fed with real domestic wastewater. Three laboratory-scale systems utilizing aerobic and anoxic zones and step feeding regime were operated for sludge age of 13, 17 and 20 days in the main reactors. A significant influence of sludge age on the sludge reduction was observed compared to conventional activated sludge systems (CAS). The greater corresponding sludge reduction was achieved as 58% operated at interchange ratio of 7.7% (1/13) in the side-stream reactor, while others revealed 37% and 35%, where interchange ratios were 5.9 (1/17) and 5.0% (1/20), respectively. In both CAS and OSA systems, high removal efficiencies of organic matter and nitrogen were achieved using real domestic wastewater. The results indicate that intermittently aerated OSA systems could accomplish less sludge production and higher nitrogen removal (up to 85%) simultaneously. Thus, it is suggested that interchange ratio of around 8% is more optimized level, which is a little lower than that of proposed and applied in most studies in the literature, which would possibly be more cost-effective.