AD-Net Research Colloquium Sept 2017 Choosing Trace Elements to Maximise Benefits (to AD)

Transcription

1 AD-Net Research Colloquium Sept 2017 Choosing Trace Elements to Maximise Benefits (to AD) Cynthia Carliell-Marquet Honorary Senior Research Fellow, University of Birmingham Senior Process Designer, Severn Trent Water

2 Member countries Ecological Functions of Trace Metals in Anaerobic Biotechnologies Ecological Functions of Trace Metals in Anaerobic Biotechnologies

3 What benefits do the right TE bring to AD? How do TE act to boost AD? Where is the proof? Where do TE sit in an AD optimisation strategy Choosing TE to maximise benefits to AD Do you have enough TE in your digester (total metals, bioavailability, speciation)? Using AD activity measurements to determine TE deficiency/benefit

4 What benefits do the right TE bring to AD? More methane per kg of feed material Better volatile solids reduction Enhanced digestate product Increased digester resilience bouncebackability Pushing boundaries on process limitations, e.g. high- ammonia digesters

5 TE act primarily on the methanogenic pathways Ni Fe Co 4. METHANOGENESIS Acetate Biogas Methane and Carbon dioxide oxidation Ni Co Carbon dioxide & Hydrogen Mo Fe W Se TE such as Co, Ni, Fe are well-known co-factors in methanogenic enzymes 3. ACETOGENESIS 2. ACIDOGENESIS Volatile Fatty Acids (butyrate, lactate, acetate); ethanol Amino acids, sugars Large carboxylic acids and alcohols also, Mo, W, Se, Cu, Zn.. Complex organic material into digester 1. HYDROLYSIS Protein Carbohydrate Lipid Complex particulate material

6 Where is the proof? Many papers published, this 2015 review collates them

7 TE contribute to an AD optimisation strategy

8 Approximate order of mass in methanogenic cells How do you know if you have the optimum TE mix in your AD system? Total metals Complex matrices Speciation Bioavailability Fe, Zn, Ni, Co, Cu, Se, Mo, W, Mn, V This Photo by Unknown Author is licensed under CC BY- ND

9 Enzyme Speciation & bioavailability why are they important? Bulk solution Bacterial cell Organic ligands 1. Complexation Metal ligand complex (ML) Organic ligands ATP driven specific uptake M Metals in feedstock Free metal ions (M) Bio-Uptake M Open gate non-specific uptake M internal 2. Precipitation Inorganic anions (S 2-, PO 4 3-, CO 3 2- ) Metal precipitates (MS, MHPO 4, MCO 3 ) Reservoir ML or M efflux Storage Vacuole 2. Adsorption Adapted from Worms et al (2006) Biochimie v88, pp

10 How do you know if you have the right TE in your AD using microbial activity measurements?

11 Simple off-line BMP test for the maximum acetate utilisation rate of a digester

12 The BMP experimental set up Controls: BMP with no trace metals apart from those already in the digested sludge and supernatant Experiments: BMP with supplemented trace metals in increasing concentrations x3 Measure methane volume x5 Biogas Biogas Acetate Substrate not limiting Acetate 60 mm 60 mm Innoculum (0.5 g VS) Innoculum (0.5 g VS)

13 Example of TE supplements stimulating conversion of acetate to methane Digester A Digester A Rate ml/h CH 4 at STP % % % September August May March February Control TM mg/l in BMP tests Fe Co Mn Ni Zn Mo B Cu Se

14 1 Measure 2 Assess full-scale status 3 Determine potential of TE to increase digester capacity

15 Understanding BMP response to TM supplementation Putting it all together: digester diagnostics for TE optimisation acetate methane Digesters sampled 1 BMP Methane (ml at STP) Time (hours) Operational parameters recorded 2 3 Microbiological analysis qpcr, pyrosequencing etc. Full chemical analysis: cations, anions; inorganic fractionation environmental analysis Methanogens (abundance, type) Equilibrium speciation modelling: (Minteq, Phreeqc )to understand metal behaviour in BMP test

16 Conclusions The right TE are essential for digester resilience and optimisation. Measuring digester total TE concentration gives you the extent of the TE reservoir but not what is readily bioavailable. Soluble TE concentration in a digester give you an indication of the strength of the complexation system versus precipitation. High rate digesters (rapid HRT and longer SRT, e.g. UASB, AnMBR) need to retain trace elements in the solids reservoir for dosing to be economically viable hence tend to dose metal salts. CSTR digesters (HRT = SRT) might benefit from dosing chelated trace elements, as long as these are bioavailable. Acetate uptake rate BMP tests + TE supplements are a simple and rapid indication of whether a digesters rate of acetate conversion could be improved by TE dosing.

17 Thank you for listening

18 Required bio-uptake rates to maintain optimum performance Further research to optimise trace metal dosing of anaerobic digesters The grand challenge: quantifying the bio-uptake rates required to sustain optimum digester performance and quantifying the impact of metal speciation on bioavailability of dosed trace metals Determine which microbial species are present, their relative dominance and their trace metal requirements Model trace metal behaviour using equilibrium speciation models: Understand and model the anaerobic biochemical system Trace metal bioavailability in the digester