J.L. Campos

Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays

The application of anaerobic digestion technology is growing worldwide because of its economic and environmental benefits. As a consequence, a number of studies and research activities dealing with the determination of the biogas potential of solid organic substrates have been carrying out in the recent years. Therefore, it is of particular importance to define a protocol for the determination of the ultimate methane potential for a given solid substrates. In fact, this parameter determines, to a certain extent, both design and economic details of a biogas plant. Furthermore, the definition of common units to be used in anaerobic assays is increasingly requested from the scientific and engineering community. This paper presents some guidelines for biomethane potential assays prepared by the Task Group for the Anaerobic Biodegradation, Activity and Inhibition Assays of the Anaerobic Digestion Specialist Group of the International Water Association. This is the first step for the definition of a standard protocol.

Nitrification in saline wastewater with high ammonia concentration in an activated sludge unit

A nitrifying activated sludge reactor fed with a high salinity medium was operated efficiently at ammonia loading rates between 1 and 4 g NH4+ -N l(-1) d(-1). The system became completely inefficient at inlet salt concentrations higher than 525 mM due to the mixed inhibition effect of salts and ammonia. The final product was mainly nitrate although dissolved oxygen limitations caused sporadic ammonia and nitrite accumulations. Specific nitrifying activity decreased due to the saline effect. A set of activity tests showed that in the continuous reactor non-adapted biomass is rather more sensitive than biomass to the saline effect. Physical properties of biomass in the reactor (sludge volumetric index and zone settling velocity) were not affected by the saline concentration, a biomass concentration of 20 gVSS l(-1) was achieved.

Short- and long-term effects of ammonium and nitrite on the Anammox process

Autotrophic anaerobic ammonium oxidation (Anammox) is a biological process in which Planctomycete-type bacteria combine ammonium and nitrite to generate nitrogen gas. Both substrates can exert inhibitory effects on the process, causing the decrease of the specific activity of the biomass and the loss of the stable operation of reactors. The aim of the present work is to evaluate these effects in short- and long-term experiments. The short-term effects were carried out with two different types of Anammox biomass, biofilm on inorganic carriers and flocculent sludge. The effects of ammonium on both kinds of biomass were similar. A decrease of the Specific Anammox Activity (SAA) of 50% was observed at concentrations about 38 mg NH(3)-N·L(-1), while 100 mg NH(3)-N·L(-1) caused an inhibition of 80%. With regards to nitrite, the SAA was not affected at concentrations up to 6.6 μg HNO(2)-N·L(-1) but it suffered a decrease over 50% in the presence of 11 μg HNO(2)-N·L(-1) in the case of the biofilm. The flocculent biomass was much less resistant and its SAA sharply decreased up to 30% of its initial value in the presence of 4.4 μg HNO(2)-N·L(-1). The study of the long-term effects was carried out in lab-scale Sequencing Batch Reactors (SBR) inoculated with the biofilm biomass. Concentrations up to 20 mg NH(3)-N·L(-1) showed no effects on either reactor efficiency or biomass activity. However, when free ammonia concentrations reached values between 35 and 40 mg NH(3)-N·L(-1), the operation turned unstable and the efficiency was totally lost. Nitrous acid concentrations around 1.5 μg HNO(2)-N·L(-1) caused a loss of the efficiency of the treatment and a destabilization of the system. However, a total restoration of the SAA was observed after the stoichiometric feeding was applied to the SBR.

Treatment of anaerobic sludge digester effluents by the CANON process in an air pulsing SBR

The CANON (Completely Autotrophic Nitrogen removal Over Nitrite) process was successfully developed in an air pulsing reactor type SBR fed with the supernatant from an anaerobic sludge digester and operated at moderately low temperatures (18-24 degrees C). The SBR was started up as a nitrifying reactor, lowering progressively the dissolved oxygen concentration until reaching partial nitrification. Afterwards, an inoculation with sludge containing Anammox biomass was carried out. Nitrogen volumetric removal rates of 0.25 g NL(-1)d(-1) due to Anammox activity were measured 35 d after inoculation even though the inoculum constituted only 8% (w/w) of the biomass present in the reactor and it was poorly enriched in Anammox bacteria. The maximal nitrogen removal rate was of 0.45 g NL(-1)d(-1). By working at a dissolved oxygen concentration of 0.5 mg L(-1) in the bulk liquid, nitrogen removal percentages up to 85% were achieved. The reactor presented good biomass retention capacity allowing the accumulation of 4.5 g VSS L(-1). The biomass was composed by ammonia oxidizing bacteria (AOB) forming fluffy structures and granules with an average diameter of 1.6mm. These granules were composed by Anammox bacteria located in internal anoxic layers surrounded by an external aerobic layer where AOB were placed.

Applications of Anammox based processes to treat anaerobic digester supernatant at room temperature

The supernatant of an anaerobic digester was treated at 20 degrees C in two systems. The first one is a two units configuration, conformed by two sequencing batch reactors (SBR), carrying out partial nitrification and Anammox processes, respectively. Partial nitrification was achieved by granular biomass with a mean diameter of 3 mm, operating at a dissolved oxygen concentration of 2.7 mg/L. The combined system allowed the removal of nitrogen loading rates around 0.08 g N/(Ld). Afterwards, Anammox biomass was spontaneously developed in the inner core of the nitrifying granules of the SBR and therefore, partial nitrification and Anammox process were carried out in a single unit. Once the stable CANON process was established, a mean nitrogen removal rate of 0.8 g N/(Ld) was registered. The settling velocities of the granules ranged from 70 to 150 m/h with sludge volumetric index values lower than 50 mL/g VSS during the whole operation.

Nitrification at high ammonia loading rates in an activated sludge unit

Abstract Effective nitrification of ammonia at high nitrogen loading rates, up to 7.5 kg NNH + 4 m −3 d −1 was obtained in a nitrifying activated sludge unit working at a hydraulic retention time of 1.3 h and 20°C. Bicarbonate, used as a carbon source for autotrophic microorganisms, allowed maintainance of a system very stable against different shocks due to point deficiencies of oxygen or other accidents. Ammonia conversion to nitrate was normally between 97 and 99.9%. Temporary accumulations of nitrite were quickly transformed into nitrate. The relative low specific conversion rate of the biomass in the reactor (0.5–0.7 g-NNH + 4 g-VSS −1 d −1 ) was compensated by the capacity of the unit to retain high concentrations of biomass (up to 15 g-VSS l −1 ) due to the excellent characteristics of the biomass (Sludge Volume Index of 12 ml g-VSS −1 ; Zone Settling Velocity of 9 m h −1 ). Present results show that activated sludge units can be operated efficiently as a high-rate nitrifying technology. This well-known, robust and easy to operate technology is a clear alternative to the airlift or fluidised-bed systems, which can be applied for the nitrification of high load wastewater at full scale.

Autotrophic nitrogen removal at low temperature

In this work the autotrophic nitrogen removal was carried out at moderately low temperatures using two configurations: a) two-units one comprising a SHARON reactor coupled to an Anammox SBR and b) single-unit one consisting of a granular SBR performing the CANON process. At 20°C the two-units system was limited by the Anammox step and its nitrogen removal capacity was around ten times lower than the CANON system (0.08 g N/(L d) versus 1 g N/(L d)). When the CANON system was operated at 15°C the average removed nitrogen loading rate decreased to 0.2 g N/(L d). The CANON system was operated in order to limit the ammonia oxidation rate to avoid nitrite inhibition of Anammox bacteria. Since both, temperature and dissolved oxygen (DO) concentration regulate ammonia oxidizing bacteria activity, once the temperature of the reactor is decreased the DO concentration must be decreased to avoid the deeper oxygen penetration inside the granule which could cause inhibition of Anammox bacteria by oxygen and/or nitrite.

Treatment of saline wastewater in SBR aerobic granular reactors

Fish canning effluents characterized by their salt content, up to 30 g NaCl/L, were treated, previously diluted to desired concentration, in a SBR where aerobic granular sludge was produced. The formation of mature aerobic granules occurred after 75 days of operation with 3.4 mm of diameter, SVI of 30 mL/g VSS and density around 60 g VSS/L-granule. Treated organic loading rates were up to 1.72 kg COD/(m3.d) with fully organic matter depletion. Ammonia nitrogen was removed via nitrification-denitrification up to 40% when nitrogen loading rates were of 0.18 kg N/(m3.d). The presence of salt in the treated effluent did not cause a detrimental effect on the operation of the reactor once the aerobic granules were formed.

Ozonation strategies to reduce sludge production of a seafood industry WWTP

In this work, several alternatives related to the application of ozone in different streams of a seafood industry WWTP were evaluated to minimize the production of waste sludge. The WWTP was composed of two coagulation-flocculation units and a biological unit and generated around of 6550 kg/d of sludge. Ozone was applied to sludge coming from flotation units (110 g TSS/L) at doses up to 0.03 g O(3)/g TSS during batch tests, no solids solubilization being observed. Ozone doses ranging from 0.007 to 0.02 g O(3)/g TSS were also applied to the raw wastewater in a bubble column reaching a 6.8% of TSS removal for the highest ozone dose. Finally, the effect of the pre-ozonation (0.05 g O(3)/g TSS) of wastewater coming from the first flotation unit was tested in two activated sludge systems during 70 days. Ozonation caused a reduction of the observed yield coefficient of biomass from 0.14 to 0.07g TSS/g COD(Tremoved) and a slight improvement of COD removal efficiencies. On the basis of the capacity for ozone production available in the industry, a maximum reduction of sludge generated by the WWTP of 7.5% could be expected.

Simultaneous methanogenesis and denitrification of pretreated effluents from a fish canning industry.

A lab-scale hybrid upflow sludge bed-filter (USBF) reactor was employed to carry out methanogenesis and denitrification of the effluent from an anaerobic industrial reactor (EAIR) in a fish canning industry. The reactor was initially inoculated with methanogenic sludge and there were two different operational steps. During the first step (Step I: days 1-61), the methanogenic process was carried out at organic loading rates (OLR) of 1.0-1.25 g COD l-1 d-1 reaching COD removal percentages of 80%. During the second step (Step II: days 62-109) nitrate was added as KNO3 to the industrial effluent and the OLR was varied between 1.0 and 1.25 g COD l-1 d-1. Two different nitrogen loads of 0.10 and 0.22 g NO3(-)-N l-1 d-1 were applied and these led to nitrogen removal percentages of around 100% in both cases and COD removal percentages of around 80%. Carbon to nitrogen ratio (C:N) in the influent was maintained at 2.0 and eventually it was increased to 3.0, by means of glucose addition, to control the denitrification process. From these results it is possible to establish that wastewater produced in a fish canning industry can be used as a carbon source for denitrification and that denitrifying microorganisms were present in the initially methanogenic sludge. Biomass productions of 0.23 and 0.61 g VSS:g TOC fed for Steps I and II, respectively, were calculated from carbon global balances, showing an increase in biomass growth due to denitrification.