Self-management of the anaerobic digestion process

Investing in laboratory instruments and personnel training or paying external consultants?

by Mario Alejandro Rosato

Managing a biogas plant does not require personnel trained in biology or chemistry. It requires only an open mind to accept becoming a “bacteria farmer” and learning the peculiarities of a new job. A good dose of Cartesian spirit about data from the literature is necessary too since conclusions from peer-reviewed papers not always are applicable to particular cases. Like chicken, pigs or cattle, bacteria will be highly productive if they are given the optimum conditions to thrive… which is not usually the case. The reason is that most biogas plant constructors base their commercial strategy on standardized designs. The result is sub-optimal systems: digesters too big or too small, feeding mechanisms suitable for only one kind of biomass, etc. Very often, the biological process is inefficient because most of the popular control methods are too simplistic. Such methods rely on tables of physic-chemical parameters, easy to measure in the field but unable to provide a reliable indication of the biological activity and the feedstock’s quality.

Anaerobic digestion has a long tradition and is the core business of the wastewater treatment and municipal waste management industries. But it is a “new” business to farmers, ancillary to their main activity. A logical consequence is that owners of agricultural biogas plants will then tend to delegate the management of the process to external companies, usually the builder or a specialized consultant. Nothing wrong with that, but such services are usually limited to:

A) Visiting the plant once a week, or even once a month, depending on the budget. During such visits, the technician takes some samples and performs some simple checks: pH, ORP, electrical conductivity, smell and colour (sic!), Volatile Fatty Acids to Total Alkalinity ratio (VFA/TA, a.k.a. with the German name FOS/TAC).

B) Sending some samples to a central laboratory – often located far away – which may measure other parameters: profile of the volatile fatty acids, total solids (TS), volatile solids (VS), COD (chemical oxygen demand), total N, NH4, C/N ratio… Chemical laboratories seldom determine the BMP (Biochemical Methane Potential) of the feedstock and when they do, they adopt Buswell’s formula, a quick method that yields too optimistic values.

The said approach arises several problems and the risk of negative consequences to the overall economy of the plant:

1) Except for pH and similar simple measurements, the laboratory results arrive several days later. So, the plant owner has paid for tests that reflect the plant’s history and not the present state.

2) In the meanwhile, the process may have been upset and the productivity lost. There are a thousand reasons for that and test results arriving one week later are useless for a diagnose.

3) The sludge is a living matter. During the transport from the plant to the laboratory, the bacterial activity continues. Consequently, the values measured in the laboratory do not necessarily reflect those that characterized the inoculum at the moment of the sampling.

4) The BMP of a biomass mixture is not always the proportional sum of its components’ BMPs. Hence, the calculation of the digester’s diet using BMP tables is not an accurate method. The result is either overfeeding the digester and wasting biomass, or not being able to produce enough CH4 to reach the plant’s nominal power.

Another critical aspect is the conflict of interests of some companies specialized in running biogas plants. All of them sell “boosters”, trace elements (a.k.a. micronutrients) enzymes and other products, usually at a high price, so their management decisions are not based on impartial strategies. The Author has found that in most cases such products represent a useless cost for the plant owner. Formulas of trace element mixtures are in the public domain from many sources (see Ref. 2 for an example) and the ingredients are available at fair prices from chemical wholesalers.

The following table is a critical analysis of some parameters currently employed for the management of anaerobic digestion processes.

Parameter

Usefulness

Measurement instrument

pH

Null. Anaerobic sludge has a big buffer capacity hence, when the pH is higher or lower than the commonly accepted thresholds, it is in general too late to avoid the process’ disruption.

pH-meter

VFA/TA ratio (a.k.a. FOS/TAC)

Scarce. The method is based on a syllogism, in which both presuppositions are false, hence the conclusion cannot be always true (Ref. 1).  The method does not provide any information on the specific biological activity of each group of microorganisms, hence the paradox: the VFA/TA ratio may be “in line with the literature”, but the bacteria are inhibited (a real case reported in Ref. 3)

Automatic titrator or a pH-meter and a burette for manual titration

Efficiency of VS degradation

High. This measurement provides with little effort and fair accuracy the percentage of the feedstock’s conversion into CH4.

Muffle oven and moisture analyser or analytic scale.

Dry matter percentage of the feedstock (A.k.a. total solids, TS)

Medium. Measuring just the dry matter percentage does not allow to discern between organic matter and ash. Furthermore, the test does not give any information on the true BMP of the sample.  Tables of BMP referred to dry matter are not reliable, especially in the case of feedstocks that can be easily adulterated.

Automatic moisture analyser or analytical scale and forced convection oven.

Volatile solids of the feedstock (VS)

High. The VS percentage of a feedstock provides a better idea of its BMP than the sole TS, although it is not enough.

Muffle oven and moisture analyser scale or laboratory scale

Residual BMP of the digestate

Very high. It provides a reliable and accurate value of the fraction of the BMP that the plant is not able to convert. It can be employed to diagnose if a digester is partly full of sediments, and the dead volume of the latter.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation

BMP of the feedstock (single biomass or mixture of several substrates)

Very high. It provides a reliable and accurate value of the feedstock’s maximum CH4 production. A reliable BMP value allows checking the potential income from the sales of biomethane or electricity, and if the price paid for the feedstock is fair. When testing a mixture of biomass, the test allows checking eventual inhibitory o synergic effects.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation

Curve of specific CH4 production vs. time

Very high. It allows programming the digester’s diet and checking if its solid retention time is bigger than the feedstock’s digestion time. By definition, the ultimate value of the curve is the feedstock’s BMP. The curve provides additional information about the process’ kinetics.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation

Hydrolytic activity of the inoculum

Very high. It is an early-warning system for detecting biological problems in progress or for diagnosing bottlenecks in the digestion process.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation. Cellulose (or paper tissue, or cotton!) as reference feedstock

Inhibition caused by the substrate

Very high. It allows measuring the maximum quantity of a given substrate that the digester can handle without inhibition risk (organic loading rate, OLR). Particularly useful when treating industrial waste or biomass of unknown origin, or fatty wastes.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation.

SMA (specific methanogenic activity)

Very high. Together with the hydrolytic activity test, it allows measuring the state of health of the bacterial ecosystem. It measures specifically the activity of the acetoclastic Archaea, which represents nearly 70% of all the CH4 produced in a digester.

Batch reactor and a system for measuring the volume of CH4 resulting from the fermentation. Sodium acetate (or vinegar!) as reference feedstock

Efficacy of additives (probiotics, enzymes, etc.)

Very high. Additives are in general very expensive, but the sellers often hyped their efficacy and the results are not worth the cost.  The Author has even found cases of additives with undesired effects, like increasing the total quantity of biogas produced, but lowering its CH4 content.

Batch reactors and a system for measuring the volume of CH4 resulting from the fermentation. One set untreated (control) and the other set with the product or sample.

Small scale simulation

Very high, but tests are expensive and time-consuming. During the start of a plant, simulating in advance the heating curve of the digester allows saving fuel oil. During steady-state operation, small scale tests allow optimizing the operational strategies.

A kit of continuous laboratory reactors (CSRT), a system for measuring the net CH4 flow and logging data.

A small laboratory, like the one depicted in photo 2, allows full and accurate control of the biological process in real-time. Self-management of the biological process saves the cost of third parties’ services, useless additives, and unforeseen disruptions. Checking the quality of the feedstock prevents buying adulterated biomass.

Like any other technique, optimizing the anaerobic digestion process with data from on-site biological tests requires some training of the personnel, perseverance, and attention during the test preparation. In general, 8 hours of theoretical training and some laboratory practice are enough to learn how to measure the BMP or the biological activity.

According to the Author’s experience in Italy and Spain, the investment in a mini-laboratory is in the range 7.000 € -16.000 €, reaching up to 25.000 € for a full laboratory like the one shown in photo 2. In the case of a biogas plant with 1 MW electric power, such an investment represents less than 0,5% of the total construction cost but allows consistent savings in management and biomass purchase. It usually pays back in less than one year.

References
[1] Mario A. Rosato, Managing Biogas Plants, a practical guide, chapter 6. CRC Press , August 22, 2017 
ISBN 9781138626614 – CAT# K31875.

[2] Mario A. Rosato, Quick restart of a biogas plant and micronutrients cost optimization

[3] Mario A. Rosato, Re-dimensioning the importance of the VFA/TA (FOS/TAC) method

Photo 1: Low-cost instrument for the self- management of the anaerobic digestion process: twin batch reactor, CO2 trap, flow meter with real-time normalization of the CH4 volume, manual reading and data processing.

Photo 2: Complete on-site laboratory: fully automatic BMP measurement instrument (15 batch reactors, CO2 traps, real-time normalization of the CH4 volume, datalogging in Excel format, and Internet connectivity), muffle oven, moisture analyser, and spectrophotometer.

Photos by the Author.

One thought on “Self-management of the anaerobic digestion process

  1. Dry anaerobic digestion requires minimal feedstock processing and no water addition prior to digestion. However, in addition to oil water separation, the pretreatment stage must provide mechanisms to remove water. Mechanical turnover is required inside the digester to ensure uniform fermentation and maintain the digestion pile temperature. Dry digesters are biologically self-heated through the air system and recirculation of the liquid percolating through the material. The liquid percolating in dry digesters contains the necessary biological constituents and proper pH balance to maintain good digestion conditions. Since air is used in the system, the quality of biogas is not as good as that from the wet anaerobic digestion.

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