Activated sludge diffusion for odour removal --effects of H2S on the biomass.

The removal of odours from wastewater treatment plants through diffusion of odour-containing air volumes into the aerated basins was investigated in a bench scale experimental campaign which lasted more than 200 days. Hydrogen sulphide was selected as a model odorous compound and its removal efficiencies were experimentally evaluated along with its effects on the biomass and on the main biochemical processes. Two bench scale sequencing batch reactors were fed in parallel on real primary sewage and monitored for chemical oxygen demand removal, nitrification and denitrification. The balance of H2S was also monitored after adding to one of them a Na2S liquid solution of 17 mgS l(reactor)(-1) d(-1), corresponding to a gas-phase concentration of 240 mgS (Nm3)(-1). Results showed an average sulphide removal of 94% in the reactor supplied with Na2S. Moreover, microbial composition did not show relevant variations after the addition of sulphide, and the good features of activated sludge flocs were maintained also in terms of sludge settleability. No relevant effects of sulphide were detected on carbon and nitrogen metabolism and chemical oxygen demand removal, nitrification and denitrification efficiencies were always above 75%, 95%, and 50% respectively, and comparable across the two reactors.

Combined chemical and biological degradation of tannery wastewater by a periodic submerged filter (SBBR).

The paper reports on the results of an investigation aimed to evaluate the performances of an innovative tannery wastewater process based on the combining biological degradation, carried out in a sequencing batch biofilm reactor, with chemical oxidation, performed by ozone. The combined treatment was carried out at the laboratory scale on real primary effluent coming from a centralised plant treating the wastewater from a large tanning district in Northern Italy. SBBR performances with and without ozonation were compared resulting to be very satisfactory only in the latter instance where recorded COD, NH4-N and TSS average removals were 97%, 98% and 99.9%, respectively. Such efficiencies correspond to specific concentrations in treated effluent well below the limit values fixed by the in-force Italian regulations. Furthermore, it was proved that the combined process is characterised by a very low sludge production. In fact, the measured specific sludge production (0.03 kg TSS/kg COD(removed)) resulted unexpectedly much more lower than the value reported for conventional biological systems (i.e., 0.3-0.5 kg TSS/kg COD(removed)).

Sludge incineration tests on circulating fluidised bed furnace.

Results of sludge incineration tests on a demonstrative fluidised bed furnace are reported and discussed. They show that particulate, heavy metals and acidic compounds in the emissions can be easily controlled both when sludge is spiked with chlorinated hydrocarbons up to a chlorine concentration in the feed of 5%, and when the afterburner is switched off. As for organic micropollutants, polynuclear aromatic hydrocarbons (PAH) were much lower than the Italian limits of 10 microg/m3 (no limits are at present considered in the European Directives). Dioxins (PCDDs) and furans (PCDFs) in some tests exceeded the limit of 0.1 ng/m3 (TE) but the concentrations in the fly ashes were much lower, thus evidencing a possible presence of contaminants in gas phase. PAHs and PCDD/PCDFs were not depending on the afterburning operation, the presence of organic chlorine in the feed sludge and the copper addition to sewage sludge.

Inhibiting effects of chloroform on anaerobic microbial consortia as monitored by the Rantox biosensor.

The Rantox biosensor was designed for anaerobic wastewater treatment process control, and detects modifications of the feed based on the response of the acetoclastic methanogens contained in the sensor to periodic pulses of a concentrated organic substrate. The biosensor was tested under various operating conditions at the laboratory scale, in parallel with a digester under control fed on the same substrate. The aim was to evaluate the response of the biosensor in the presence of an incoming organic toxic compound (CHCl3). The experimental set-up, i.e. the biosensor and the digester, was connected to an automated control system developed under LabVIEW environment for data acquisition and operational sequence programming (the Rantox Virtual Instrument). Biomasses with different activities were used as inocula, and inhibition was induced by dosing chloroform according to two different procedures. The results showed good sensitivity and rapid response of the biosensor to feed intoxication. The presence of chloroform was detected by the Rantox with a rapid and visible response, and well in advance with respect to the digester.

Monitoring toxicity in anaerobic digesters by the rantox biosensor: theoretical background.

Process control of anaerobic reactors is difficult due to the complexity of the methabolic pathways in the microbial consortium and to the difficulty of detecting and monitoring process instability in short time, before the biomass is poisoned by incoming toxicants. Process control based on the Rantox biosensor is based on the following principle: the wastewater that can potentially induce an overload or contains a toxicant is first tested on a small "upstream" digester (the Rantox). This reactor makes possible to detect the potential instability and, if necessary, to divert the concentrated and/or contaminated wastewater to a buffer tank and consequently to protect the active biomass of the full-scale reactor. It is generally accepted that methanogens are the most sensitive microorganisms in anaerobic digestion. Among these bacteria, the acetoclastic methanogens are of primary importance because some 70% of the converted chemical oxyen demand (COD) mass flow passes through acetic acid. Therefore the first objective in the development of the Rantox biosensor has been to monitor the metabolism of acetoclastic methanogens in the presence of toxicants. This article presents the theoretical background required to evaluate the toxicity effects by determining the kinetic constants of the considered microorganisms from experimental data. The results of two series of calibration tests, performed in order to obtain a preliminary evaluation of the biosensor response to overload and toxicity conditions, are reported. In a second article, calibration tests will be described which refer to two prototypes of the biosensor tested in different operating conditions. The crucial point related to the Rantox, i.e., its comparison with a "normal" laboratory-scale digester to simulate a full-scale plant, will be the subject of the third (and last) article, which is planned to describe the development of this instrument.