Electricity generation from bio-treatment of sewage sludge with microbial fuel cell

A two-chambered microbial fuel cell (MFC) with potassium ferricyanide as its electron acceptor was utilized to degrade excess sewage sludge and to generate electricity. Stable electrical power was produced continuously during operation for 250 h. Total chemical oxygen demand (TCOD) of sludge was reduced by 46.4% when an initial TCOD was 10,850 mg/l. The MFC power output did not significantly depend on process parameters such as substrate concentration, cathode catholyte concentration, and anodic pH. However, the MFC produced power was in close correlation with the soluble chemical oxygen demand (SCOD) of sludge. Furthermore, ultrasonic pretreatment of sludge accelerated organic matter dissolution and, hence, TCOD removal rate in the MFC was increased, but power output was insignificantly enhanced. This study demonstrates that this MFC can generate electricity from sewage sludge over a wide range of process parameters.     

The effect of temperature fluctuations on psychrophilic anaerobic sequencing batch reactors treating swine manure

Under northern climatic conditions, a temporary decrease in the temperature of anaerobic reactors treating swine manure is likely to happen at the farm. The objective of this study was to evaluate the impact of temperature fluctuations, between 10 and 20 degrees C, on the stability and performance of psychrophilic anaerobic sequencing batch reactors (ASBRs) treating swine manure. Methane yield decreased from 0.266+/-0.014 l/g of total chemical oxygen demand (TCOD) fed to the ASBRs at 20 degrees C to 0.218+/-0.022 and 0.080+/-0.002 l/g TCOD (fed) at 15 and 10 degrees C, respectively. Soluble chemical oxygen demand (SCOD) reduction decreased from 94.2+/-1.1% at 20 degrees C to 78.8+/-3.0% at 15 degrees C and 60.4+/-6.4% at 10 degrees C. Total COD removal also tended to decrease as temperature was lowered, but difference between operating temperatures was not as pronounced. A lower methanogenic activity in the ASBRs operated at 10 degrees C probably favoured quiescent conditions during the settling period, thereby increasing physical removal of the TCOD through sedimentation of the solids with the biomass. When the operating temperature was increased back to 15 and 20 degrees C, methane yield and SCOD reduction improved, but reactor performance remained significantly (P<0.05) lower than that achieved before the cycles at 10 degrees C. Results from this experiment nevertheless suggested that fluctuation in the operating temperature of psychrophilic ASBRs should only have temporary effects on the performance and stability of the process.     

Start-up alternatives and performance of an UASB pilot plant treating diluted municipal wastewater at low temperature

Different start-up procedures of an upflow anaerobic sludge bed (UASB) digester were carried out. Start-up without inoculum (experiment A) was delayed for about 120 day. The digester reached 75-85% total suspended solids (TSS) removal, 54-58% total chemical oxygen demand (TCOD) removal and 63-73% biological oxygen demand (BOD5) removal at influent concentrations of 240-340 mg TCODil-1, temperatures of 13.5-15 degrees C and hydraulic retention times (HRT) of 10-11 h. Digested sludge was used as inoculum in experiment B. After the start-up period of 75 days, digester efficiencies were 58%, 41% and 54% for TSS, TCOD and BOD5 removal, respectively, working at 169 mg TCODil-1, temperature of 14 degrees C and HRT of 11 h. The sludge bed developed and stabilised quickly when using a hydraulically adapted inoculum (experiment C), but TCOD and BOD5 removals remained low and volatile fatty acids (VFA) accumulated in the effluent.     

Anaerobic treatment of low-strength municipal wastewater by a two-stage pilot plant under psychrophilic conditions

A two-stage anaerobic treatment pilot plant was tested for the treatment of raw domestic wastewater under temperatures ranging from 21 to 14 degrees C. The plant consisted of a hydrolytic upflow sludge bed (HUSB) digester (25.5m3) followed by an upflow anaerobic sludge blanket (UASB) digester (20.36m3). The hydraulic retention time (HRT) varied from 5.7 to 2.8h for the first stage (HUSB digester) and from 13.9 to 6.5h for the second stage (UASB digester). Total suspended solids (TSS), total chemical oxygen demand (TCOD), and biochemical oxygen demand (BOD) removals ranged from 76% to 89%, from 49% to 65%, and from 50% to 77%, respectively, for the overall system. The percentage of influent COD converted to methane was 36.1%, the hydrolysis of influent volatile suspended solids (VSS) reached 59.7% and excess biomass was 21.6% of the incoming VSS. Plant performance was influenced by the wastewater concentration and temperature, yet better results were obtained for influent COD higher than 250mg/l.     

Enhanced anaerobic gas production of waste activated sludge pretreated by pulse power technique

An electric pulse-power reactor consisting of one coaxial electrode and multiple ring electrodes was developed to solubilize waste activated sludge (WAS) prior to anaerobic digestion. By pretreatment of WAS, the soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) ratio and exocelluar polymers (ECP) content of WAS increased 4.5 times and 6.5 times, respectively. SEM images clearly showed that pulse-power pretreatment of WAS was found to result in destruction of sludge cells. Batch-anaerobic digestion of pulse-power treated sludge showed 2.5 times higher gas production than that of untreated sludge. Solubilized sludge cells by pulse-power pretreatment would be readily utilized for anaerobic microorganisms to produce anaerobically-digested gas. Slow or lagged gas production in the initial anaerobic digestion stage of pulse-power pretreated sludge implied that the methane-forming stage of anaerobic digestion would be the rate-limiting step for anaerobic digestion of pulse-power pretreated sludge.     

Performance evaluation and kinetic modeling of the start-up of a UASB reactor treating municipal wastewater at low temperature

A kinetic modeling-based study was carried out to evaluate the start-up performance of a 10-L up-flow anaerobic sludge blanket (UASB) reactor treating municipal wastewater under different organic and hydraulic loading conditions. The reactor was operated for 105 days (around 4 months) below 20 °C and with three different hydraulic retention times of 24, 12 and 5 h. Imposed volumetric organic loading rates (OLR) ranged from 0.57 (±0.05) to 11.78 (±0.85) kg TCOD/m³-day. Although relatively high incoming volumetric OLR values were employed to the system, the UASB reactor demonstrated a favorable performance on the anaerobic treatability of municipal wastewater, and no process failure was recorded in the start-up stage. On the basis of experimental results, the modified Stover–Kincannon model was successfully applied to define the start-up kinetics with a very high value of the correlation coefficient (R = 0.9729). Maximum substrate utilization rate constant and saturation constant of the modified Stover–Kincannon model were determined as U _max = 1.996 g/L-day and K _B = 1.536 g/L-day, respectively.     

Treatment of wastewater from abattoirs before land application--a review

Pre-treatments are screening, catch basins, flotation, equalization, and settlers for recovering proteins and fats from abattoir wastewater. With chemical addition, dissolved air flotation (DAF) units can achieve chemical oxygen demand (COD) reductions ranging from 32% to 90% and are capable of removing large amounts of nutrients. Aerobic trickling towers reduced soluble COD by additional 27% but did not reduced total COD. Chemical-DAF reduced 67% of total COD and soluble COD. About 40-60% of the solids or approximately 25-35% of the biological oxygen demand (BOD) load can be separated by pre-treatment screening and sedimentation. Anaerobic systems are lagoon, anaerobic contact (AC), up-flow anaerobic sludge blanket (UASB), anaerobic sequence batch reactor (ASBR), and anaerobic filter (AF) processes. Abattoir wastewater is well suited to anaerobic treatment because it is high in organic compounds. Typical reductions of up to 97% BOD, 95% SS and 96% COD are reported. UASB's average COD removal efficiencies are of 80-85%. UASB seems to be a suitable process for the treatment of abattoir wastewater, due to its ability to maintain a sufficient amount of viable sludge. Wastewater in abattoirs can be reduced by treatment of immersion chiller effluent by membrane filtration which can produce recyclable water. Total organic C can be reduced below 100mg/L, and bacteria can not pass through the membrane pores. The abattoir waste minimization options are also discussed.     

Partial nitrification and nutrient removal in intermittently aerated sequencing batch reactors treating separated digestate liquid after anaerobic digestion of pig manure

The performance of an intermittently aerated sequencing batch reactor (IASBR) technology was investigated in achieving partial nitrification, organic matter removal and nitrogen removal from separated digestate liquid after anaerobic digestion of pig manure. The wastewater had chemical oxygen demand (COD) concentrations of 11,540 ± 860 mg/L, 5-day biochemical oxygen demand (BOD₅) concentrations of 2,900 ± 200 mg/L and total nitrogen (TN) concentrations of 4,041 ± 59 mg/L, with low COD:N ratios (2.9) and BOD₅:COD ratios (0.25). Synthetic wastewater, simulating the separated digestate liquid with similar COD and nitrogen concentrations but BOD₅ of 11,500 ± 100 mg/L, was also treated using the IASBR technology. At a mean organic loading rate of 1.15 kg COD/(m³ d) and a nitrogen loading rate of 0.38 kg N/(m³ d), the COD removal efficiency was 89.8% in the IASBR (IASBR-1) treating digestate liquid and 99% in the IASBR (IASBR-2) treating synthetic wastewater. The IASBR-1 effluent COD was mainly due to inert organic matter and can be further reduced to less than 40 mg/L through coagulation. The partial nitrification efficiency of 71–79% was achieved in the two IASBRs and one cause for the stable long-term partial nitrification was the intermittent aeration strategy. Nitrogen removal efficiencies were 76.5 and 97% in IASBR-1 and IASBR-2, respectively. The high nitrogen removal efficiencies show that the IASBR technology is a promising technology for nitrogen removal from low COD:N ratio wastewaters. The nitrogen balance analysis shows that 59.4 and 74.3% of nitrogen removed was via heterotrophic denitrification in the non-aeration periods in IASBR-1 and IASBR-2, respectively.     

Degradation and characteristic changes of organic matter in sewage sludge using microbial fuel cell with ultrasound pretreatment

Microbial fuel cell (MFC) could be an efficient sludge treatment unit in regard of rates and extents of total chemical oxygen demand (TCOD) removal, particularly when ultrasound was applied to pretreat the sludge. This study characterized the organic matter in sludge before and after MFC treatment, with or without ultrasound as a pretreatment stage. The 5-d MFC tests with electric load significantly enhanced TCOD removal rate from 11.3% to 19.2% for raw sludge and from 25% to 57% for sludge pretreated with >0.6 W ml^?1 ultrasound, using conventional anaerobic digestion test (without electric load) as control. The aromatic proteins, soluble microbial byproduct-like fluorescent compounds and carboxylic components, aliphatic components (C–H related), hydrocarbon and carbohydrate materials were identified to be principally released by ultrasound pretreatment and the fuels in the present MFC study.     

Application of polyethylene glycol immobilized Clostridium sp. LS2 for continuous hydrogen production from palm oil mill effluent in upflow anaerobic sludge blanket reactor

A novel polyethylene glycol (PEG) gel was fabricated and used as a carrier to immobilize Clostridium sp. LS2 for continuous hydrogen production in an upflow anaerobic sludge blanket (UASB) reactor. Palm oil mill effluent (POME) was used as the substrate carbon source. The optimal amount of PEG-immobilized cells for anaerobic hydrogen production was 12% (w/v) in the UASB reactor. The UASB reactor containing immobilized cells was operated at varying hydraulic retention times (HRT) that ranged from 24 to 6 h at 3.3 g chemical oxygen demand (COD)/L/h organic loading rate (OLR), or at OLRs that ranged from 1.6 to 6.6 at 12 h HRT. The best volumetric hydrogen production rate of 336 mL H₂/L/h (or 15.0 mmol/L/h) with a hydrogen yield of 0.35 L H₂/g COD_removed was obtained at a HRT of 12 h and an OLR of 5.0 g COD/L/h. The average hydrogen content of biogas and COD reduction were 52% and 62%, respectively. The major soluble metabolites during hydrogen fermentation were butyric acid followed by acetic acid. It is concluded that the PEG-immobilized cell system developed in this work has great potential for continuous hydrogen production from real wastewater (POME) using the UASB reactor.     

Electricity generation and treatment of paper recycling wastewater using a microbial fuel cell

Increased interest in sustainable agriculture and bio-based industries requires that we find more energy-efficient methods for treating cellulose-containing wastewaters. We examined the effectiveness of simultaneous electricity production and treatment of a paper recycling plant wastewater using microbial fuel cells. Treatment efficiency was limited by wastewater conductivity. When a 50 mM phosphate buffer solution (PBS, 5.9 mS/cm) was added to the wastewater, power densities reached 501 +/- 20 mW/m(2), with a coulombic efficiency of 16 +/- 2%. There was efficient removal of soluble organic matter, with 73 +/- 1% removed based on soluble chemical oxygen demand (SCOD) and only slightly greater total removal (76 +/- 4%) based on total COD (TCOD) over a 500-h batch cycle. Cellulose was nearly completely removed (96 +/- 1%) during treatment. Further increasing the conductivity (100 mM PBS) increased power to 672 +/- 27 mW/m(2). In contrast, only 144 +/- 7 mW/m(2) was produced using an unamended wastewater (0.8 mS/cm) with TCOD, SCOD, and cellulose removals of 29 +/- 1%, 51 +/- 2%, and 16 +/- 1% (350-h batch cycle). These results demonstrate limitations to treatment efficiencies with actual wastewaters caused by solution conductivity compared to laboratory experiments under more optimal conditions.     

Design,Construction, and Starting-up of an Anaerobic Reactor for the Stabilisation,Handling, and Disposal of Excess Biological Sludge Generated in a Wastewater Treatment Plant

Design, construction, and starting-up of an upflow anaerobic sludge blanket reactor was carried out. This system was proposed for excess sludge stabilisation, particularly that generated at an activated sludge wastewater treatment facility installed in a sugarcane mill. The upflow anaerobic sludge blanket (UASB) reactor built, had a working volume of 22.3 m³and a hydraulic residence time of 22 days. Methane production was at a maximum of 79% volume with an average of 60% for this treatment. For starting up the anaerobic reactor, a suitable inoculum from a neighboring plant was used. As the waste characteristics in both plants were different, an acclimation procedure was followed to achieve granulation. Control and stability of anaerobic reactions were monitored with alkalinity data, using the so-called ‘alfa alkalinity’ to try to keep its value at around 0.4. Once pseudosteady-state conditions were reached (chemical oxygen demand reduction and methane-rich biogas production within ±10 percent), the organic load was steadily increased up to feeding 100% excess sludge. The UASB reactor used to stabilise the excess biomass generated a sludge with a much lower volume than that originally fed. Its design ensured adequate hydraulic flow and biogas production with a high methane content. The bacteria were attached constituting spheres and very minor maintenance operations were required.     

Anaerobic degradation of azo dye Drimaren blue HFRL in UASB reactor in the presence of yeast extract a source of carbon and redox mediator

This paper presents results on anaerobic degradation of the azo dye blue HFRL in a bench scale Upflow anaerobic sludge blanket (UASB) reactor operated at ambient temperature. The results show that the addition of yeast extract (500 mg/L) increased color removal (P < 0.05) from 62 to 93% despite the low chemical oxygen demand (COD) removal (~35%) which happened due to volatile fatty acids (VFA) accumulation. There were no differences in color removal (~91%) when yeast extract (500 mg/L) was used in the presence or absence of glucose, suggesting that yeast extract acted as source of redox mediator (riboflavin) and carbon. The specific rate of dye removal increased along the operational phases and depended on the presence of yeast extract, suggesting progressive biomass acclimatization. Analysis of bacterial diversity by Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR–DGGE) method showed there was biomass selection along the bioreactor operation and no evidence of azo dye degrading bacteria predominance. This strengthens the hypothesis that color removal happens extracellularly by the reduction of azo bond by reduced redox mediators, such as riboflavin, which is present in high amount in the yeast extract.     

Enhanced sulfate reduction and trichloroethylene (TCE) biodegradation in a UASB reactor operated with a sludge developed from hydrothermal vents sediments: Process and microbial ecology

Most Trichloroethylene (TCE) biodegradation reports refer to methanogenic conditions, however, in this work, enhanced sulfidogenesis and TCE biodegradation were achieved in an upflow anaerobic sludge blanket (UASB) reactor in which a completely sulfidogenic sludge, from hydrothermal vents sediments, was developed. The work was divided in three stages, (i) sludge development and sulfate reducing activity (SRA) evaluation, (ii) TCE biodegradation and (iii) SRA evaluation after TCE biodegradation. For (i) SR was 98 ± 0.1%, 84% as sulfide (H₂S, 1200 ± 28 mg/L), sulfate reducing activity (SRA) was 188 ± 50 mg COD H₂S/g VSS*d. For (ii) The reactor reached 74% of TCE removal, concentrations of vinyl chloride of 16 ± 0.3 μM (5% of the TCE added) and ethene 202 ± 81 μM (67% of the TCE added), SRA of 161 ± 7 mg COD H₂S/g VSS*d, 68% of sulfide (H₂S) production and 93% of COD removal. For (iii) SRA was of 248 ± 22 mg COD H₂S/g VSS*d demonstrating no adverse effects due to TCE.Among the genera of the microorganisms identified in the sludge during TCE biodegradation were: Dehalobacter, Desulfotomaculum, Sulfospirillum, Desulfitobacterium, Desulfovibrio and Clostridium. To the best of our knowledge, this is the first report using a sulfidogenic UASB reactor to biodegrade TCE. The overall conclusions of this work are that the reactor is efficient on both, sulfate and TCE biodegradation and it could be used to decontaminate wastewater containing organic solvents and relatively high concentrations of sulfate.     

Performance of staged and non-staged up-flow anaerobic sludge bed (USSB and UASB) reactors treating low strength complex wastewater

The use of anaerobic processes to treat low-strength wastewater has been increasing in recent years due to their favourable performance-costs balance. For optimal results, it is necessary to identify reactor configurations that are best suited for this kind of application. This paper reports on the comparative study carried out with two high-rate anaerobic reactor systems with the objective of evaluating their performances when used for the treatment of low-strength, complex wastewater. One of the systems is the commonly used up-flow anaerobic sludge blanket (UASB) reactor. The other is the up-flow staged sludge bed (USSB) system in which the reactor was divided longitudinally into 3, 5 and 7 compartments by the use of baffles. The reactors (9 l) were fed with a synthetic, soluble and colloidal waste (chemical oxygen demand (COD) < 1000 mg/l) and operated at 28°C and 24 h hydraulic retention time. Intermediate flow hydraulics, between plug-flow and completely-mixed, in the UASB and 7 stages USSB reactors allowed efficient degradation of substrates with minimum effluent concentrations. Low number of compartments in the USSB reactors increased the levels of short-circuiting thus reducing substrate removal efficiencies. All reactors showed high COD removal efficiencies (93–98%) and thus can be regarded as suitable for the treatment of low strength, complex wastewater. Staged anaerobic reactors can be a good alternative for this kind of application provided they are fitted with a large enough (≥7) number of compartments to fully take advantage of their strengths. Scale factors seem to have influenced importantly on the comparison between one and multi staged sludge-bed reactors and, therefore, observations made here could change at larger reactor volumes.     

Influence of biogas-induced mixing on granulation in UASB reactors

Studies have been carried out to correlate biogas-induced mixing and granulation in upflow anaerobic sludge blanket (UASB) reactors, treating low-strength as well as high-strength biodegradable wastewaters. A dimensionless granulation index (GI) has been framed taking into account the mixing in sludge bed due to produced biogas. Analysis of full-scale, pilot-scale and lab-scale UASB reactors treating actual wastewaters reveals the significance of biogas-induced mixing, represented by GI, on granulation of biomass in the reactors. For obtaining proper granulation in UASB reactors (percentage granules greater than 50%, w/w), resulting in higher chemical oxygen demand (COD) removal efficiency, it is recommended to maintain GI values in the range of 15,000–57,000.     

Characteristics of simultaneous organic and nutrient removal in a pilot-scale vertical submerged membrane bioreactor (VSMBR) treating municipal wastewater at various temperatures

A pilot-scale vertical submerged membrane bioreactor (VSMBR) with anoxic and oxic zones in one reactor was operated in an attempt to reduce the problems concerning effective removal of organic matter and nutrients from municipal wastewater. Source water with total chemical oxygen demand (TCOD)/total nitrogen (TN) ratio of 5.5 was treated at various temperatures (13–25 °C) over an interval of about 1 year. As a result, total suspended solid (TSS) and TCOD were removed by 100% and higher than 98%, respectively. Moreover, the average removal efficiencies of TN and total phosphorus (TP) were found to be 74% and 78% at 8 h-hydraulic retention time (HRT) and 60-days sludge retention time (SRT). Under these conditions, the specific removal rates (SRR) of TN and TP were found to be 0.093 kg N m⁻³ day⁻¹ and 0.008 kg P m⁻³ day⁻¹, and the daily production of excess sludge (DPES), 0.058 kg TSS day⁻¹.     

An integrated anaerobic–aerobic bioreactor (IAAB) for the treatment of palm oil mill effluent (POME): Start-up and steady state performance

Palm oil mill effluent (POME) with average chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of 70,000 and 30,000 mg/L, respectively, can cause serious environmental hazards if discharged untreated. There are conventional palm oil mill effluent (POME) treatment systems that require large footprint, long HRT and fail to meet the Malaysian Department of Environment (DOE) discharge limit. Hence, the current research is aimed to design a novel integrated anaerobic–aerobic bioreactor (IAAB) for POME treatment in order to overcome these shortcomings of the conventional system. IAAB is a new bioreactor configuration which integrates anaerobic and aerobic digestion in one reactor. The overall removal efficiencies in steady state condition in terms of chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total suspended solids (TSS) were more than 99% at the organic loading rate (OLR) of 10.5 g COD/L day with methane yield of 0.24 L CH₄/g COD removed. The effluent quality remained stable (BOD < 70 mg/L) and complied with the discharge limit (BOD < 100 mg/L). Overall, the IAAB system exhibited good stability and pH adjustment was unnecessary. The results show that the IAAB achieves higher performance in terms of organic removal efficiency and methane yield at higher OLR and shorter HRT as compared to the conventional system. Further evaluations of its long-term performance are proposed for the subsequent study.     

Effect of organic loading rate on the performance of an up-flow anaerobic sludge blanket reactor treating olive mill effluent

The primary objective of this study was to evaluate the effects of the organic loading rate on the performance of an up-flow anaerobic sludge blanket (UASB) reactor treating olive mill effluent (OME), based on the following indicators: (i) chemical oxygen demand (COD) removal efficiency; and (ii) effluent variability (phenol, suspended solids, volatile fatty acids, and pH stability). The UASB reactor was operated under different operational conditions (OLRs between 0.45 and 32 kg COD/m³·day) for 477 days. The results demonstrated that the UASB reactor could tolerate high influent COD concentrations. Removal efficiencies for the studied pollution parameters were found to be as follows: COD, 47∼92%; total phenol, 34∼75%; color, 6∼46%; suspended solids, 34∼76%. The levels of VFAs in the influent varied between 310 and 1,750 mg/L. Our measurements of the VFA levels indicated that some of the effluent COD could be attributed to VFAs (principally acetate, butyrate, iso-butyrate, and propionate) in the effluent, which occurred at levels between 345 and 2,420 mg/L. As the OLRs were increased, more VFAs were measured in the effluent. A COD removal efficiency of 90% could be achieved as long as OLR was kept at a level of less than 10 kg COD/m³·day. However, a secondary treatment unit for polishing purposes is necessary to comply with discharge standards.     

Assessment of the biodegradability of a monosulfonated azo dye and aromatic amines

Biodecolourisation of an azo dye by anaerobic cultures using a liposomal textile levelling agent as primary substrate was assessed. Liposomes seem to facilitate the uptake of the dye (Acid Orange 7) by anaerobic biomass, leading to a fast decolourisation (colour removal of 96% was achieved in the first sample port of the reactor profiles). On the other hand, the presence of dye (60–300 mg l⁻¹) caused a decrease in the chemical oxygen demand (COD) degradation rate (4.1–2.5 g COD removed l⁻¹ d⁻¹ for 60 and 300 mg l⁻¹ of dye, respectively), suggesting inhibitory effects.Aerobic degradation of aromatic amines was investigated in aerobic respirometric assays with different types of inocula. Sulfanilic acid and aniline were mineralised by inocula with a significant microbiological diversity, even with domestic effluent. These results were confirmed by a significant reduction of COD, total organic carbon (TOC) and a high oxygen consumption (biochemical oxygen demand/theoretical oxygen demand), 92±4%. Kinetic analysis showed that a sigmoid function describes quite well the experimental data, even better than the exponential model. Orthanilic and metanilic acids and 1-amino-2-naphtol were persistent under the tested conditions.