Mesophilic and psychrophilic digestion of liquid manure

Research output: Thesisinternal PhD, WU

Abstract

<strong>IN GENERAL</strong><p>In this thesis the possibilities for digestion of cow and pig manure are described for a completely stirred tank reactor system (CSTR) and an accumulation system (AC-system).<br/>For this purpose were researched:<br/>1. Anaerobic digestion of cow manure. Optimization of the digestion process for energy production on dairy farms.<br/>2. Digestion of manure at lower temperatures.<p>The goal of the first mentioned research was optimization of anaerobic digestion of cow manure in a mesophilic CSTR-system. The results of this research as well as practice experience show that without considerable problems a stable anaerobic digestion process is obtained. Further application of anaerobic digestion of animal slurries on farm-scale in CSTR-systems under mesophilic conditions is at the present energy prices economically not remunerative. This was an important reason for starting research on the applicability of simpler and cheaper systems for digesting manure, such as an accumulation system (AC- system) at lower temperatures.<p>A major part of research on manure digestion is carried out in mesophilic conditions and also in practice the application is mostly in the mesophilic temperature range (Demuynck <em>et al.,</em> 1984). Results of laboratory research (v. Velsen, 1981; Hawkes <em>et al.</em> , 1984; Yaldiz, 1987; Hill <em>et al.</em> , 1983) offered little perspective for application of low temperature digestion in practice. Results in this thesis show that the retention times in the above mentioned research were to short to reach a stable digestion at lower temperatures.<p>The goal of the research on digestion of manure at lower temperatures was to obtain insight into processes concerning anaerobic digestion of manure at low temperatures and to investigate the applicability of digestion and storage in a so-called AC-system.<p>A CSTR-system is characterized by a constant supply of fresh manure as well as by a constant removal of digested manure. The sludge load is therefore constant. An AC-system is also characterized by a constant supply of fresh manure, but the removal of digested manure only takes place once per filling (storage) period. The small inoculation at the start of the filling period, typical for an AC-system, results in a high initial sludge load. Through the growth of the biomass during the filling time the sludge load will decrease.<p><strong>START-UP OF SLURRY DIGESTION SYSTEMS.</strong><p>It is clear that the first start-up should be carried out with inoculation material as much as possible adapted to the digestion conditions. However such inoculation material is hardly ever available. It is therefore important to know the factors affecting start-up and the stability of the ultimate digestion process, such as:<br/>- process temperature.<br/>- the quality of the inoculation material.<br/>- the amount available inoculation material.<br/>- sludge load.<p>Chapter 2 and 3 of this thesis describe the start-up of manure digestion in several systems. The results presented in Chapter 2 show that with batch wise digestion of fresh manure without inoculation at 5, 10 and 15°C no methane is produced within a 5 months period. Methane production at these low temperatures is possible when high inoculation percentages are applied. A 50% inoculation shows even at a process temperature as low as 5°C methane production.<p>Systems, also without inoculation, at process temperatures of 20°C and higher will obtain in a relatively short time sufficient methanogenic activity. With batch wise digestion of fresh cow manure at 25 and 30°C the lag-phase is respectively ± 60 and 20 days, while the accumulated fatty acids are converted into methane in 125 en 75 days. With a non-inoculated manure digestion start-up the quality of the manure to be digested is of considerable importance. When manure has been stored for some time a sort of pre-start- up has taken place. This explains the relative short lag-phase (respectively ± 20 and 60 days) at the start- up of an AC-system for digesting stored cow and pig manure at 20°C.<p>The results in Chapter 2 also show that a non-inoculated start-up at temperatures of 15°C and lower, will cover an extended period of time. High inoculation provides a fast start-up, even at low temperatures, but is mostly not feasible in practice.<p>Chapter 3 describes the realization of start-up of an AC-system at 15°C with low inoculation (1-13%). The quality of the inoculation material appears to be of great importance for the proceeding of the start- up. This quality is highly dependable on the cultivation conditions, viz.:<br/>- The type of digestion system used.<br/>- The applied process temperature.<p><u>The used digestion system</u><br/>Research results show that digested manure from a CSTR-system is not suitable for starting-up an AC-system at 15°C 1-13% inoculation and 100 days filling time. In order to remove accumulated fatty acids when starting-up an AC-system on cow manure, extreme long digestion times are required (240-310 days), under the above mentioned conditions and inoculated with at 20°C digested CSTR-system manure (20°C -CSTR-sludge). At a following 'second start-up', 13% inoculated with the 'freshly' digested material, the start-up period is only 150 instead of 240 days. The 'first start-up' is characterized by a successive degradation of acetic and propionic acid, whereas in the 'second start-up' with AC-sludge these fatty acids are removed simultaneously. Similar results can be achieved with the start-up of an AC-system at 15°C for the digestion of pig manure with 15°C-CSTR-sludge as inoculation.<p><u>The applied process temperature</u><br/>The quality of the inoculation material is not only influenced by the system in which it was cultivated but also by the temperature it was liable to. The starting-up of an AC-system at 15°C will proceed faster with a 15% inoculation of 15°C-AC-sludge than with 20°C-AC-sludge. Similar results have been found with batch digestion of manure. Inoculation with sludge cultivated at 18°C will give a higher gas production in batch digestion at 20°C than inoculum cultivated at 27 and 35°C. At process temperatures of 27 and 35°C. the mentioned inocula show little difference in gas production. Wellinger and Kaufmann (1982) and Cullimore <em>et al.</em> (1985) also found this acclimatization of the sludge to lower temperatures.<p>When using the same amount and type of inoculum at a digestion temperature of 20°C the starting-up period in an AC-system is considerable shorter than at 15°C. At 20°C the accumulated fatty acids can be converted within the filling time of 100 days with a 7-13% inoculum, whereas at 15°C a start-up period of at least 240 days was required.<p>In practice non or little adapted inoculum is available. It is than recommendable to operate the AC- system during the first start-up at 20°C. The following filling can be conducted at 15°C In practice this means AC-systems should be started-up preferably in summer or additional heating should be installed. When it is impossible to raise the process temperature temporarily, additional storage capacity should be available, in order to convert the accumulated fatty acids and to obtain a sludge with sufficient C2 and C3 degradation capacity.<p>NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N is a major inhibition component in the digestion of manure. When sludge unadapted for high NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentrations, is used for starting-up an AC-system, an adaptation period should be taken into calculation, in which the fatty acid degradation and gas production will stagnate (v. Velsen, 1981). With the start-up of AC-systems on pig manure, Hill <em>et al.</em> (1983) attribute the inhibition of the methane production to the high concentrations of accumulated fatty acids, when sewage sludge was used as inoculum. The results in Chapter 3 indicate that when unadapted inoculum (such as sewage sludge and granular sludge) is used in an AC-system for digesting manure with a relatively high NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentration, an adaptation period is required to NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N in which a stagnation of the fatty acid degradation and gas production will occur. The results of starting-up experiments of both CSTR- as AC-systems with CSTR sludge (presented respectively in Chapter 2 and 3) indicate that also inhibition by fatty acids can appear, viz.:<br/>- When a CSTR-system at 20°C is started-up with mesophilic sludge operated at a high sludge loading, the 'steady state' is characterized by a high fatty acid concentration in the effluent and low gas production. However when the sludge loading is increased step wise, other conditions unchanged, the 'steady state' is characterized by a low fatty acid concentration in the effluent and a high gas production.<br/>- When 50% (20°C -CSTR-sludge is used as inoculum for starting-up an AC-system for treating cow manure at 15°C the lowest gas production was observed with the highest sludge loading (ft= 70 days instead of 100 days). The results of the experiments with as well the CSTR- as the AC-system show that the inhibition can be terminated by a temporary raise of the process temperature.<p><strong>PROCESS MANAGEMENT OF CSTR- AND AC-SYSTEMS.</strong><p>Results concerning continuous experiments with CSTR- and AC-systems are described in Chapter 4 and 5. The methane production in the digestion of manure depends on the course of hydrolysis, acidification and methanogenesis. Temperature and applied loading are the two most important process parameters .<p>The research concerning the digestion in completely mixed systems is carried out with cow manure in a temperature range from 15 to 40°C. It is indeed possible to obtain a stable digestion at 15°C but in this case very long retention times are required (~! 100 days). Also the gas production is then considerably lower compared with a digestion at 30°C and 20 days retention time. This lower gas production is caused by a reduced hydrolysis.<p>Digestion at 20°C and a retention time of 100 days results in a similar gas production as a digestion at 30°C and 20 days retention time. Under these conditions approximately 25% of the influent COD is converted to methane gas. About half of this origins from fatty acids already present in the influent and the rest is derived through hydrolysis of suspended solids. At longer retention times no significant increase of the specific gas production is found. With an increase to 35°C only at a retention time of 15 days a small significant increase of the gas production is observed. With process temperatures of 30-35°C the specific gas production (m3/m3 manure) increases with increasing retention times ranging from 10-20 days. The hydrolysis is the rate limiting step. The non-VFA-dissolved-COD fraction is inert to anaerobic treatment. For non of the investigated process conditions a reduction of this non-fatty acid dissolved COD is found.<p>Digestion at 30-35°C in a CSTR-system is well capable of dealing with sudden increases of the loading as a result of decreases of retention time. A one step reduction of the retention time of 15, 20, 25 or 30 to 10 leads to a direct increase of the gas production and only a minor raise of the effluent fatty acid concentration. In this way the volumetric gas production (m3/m3.day) can be more than doubled within a few days. In practice the methane production can be adapted, in this way, to the energy demand at that moment.<p>In Chapter 5 results of cow and pig manure digestion in AC-systems are presented. The course of the digestion is dependent on process temperature, filling time, percentage inoculation and the composition and concentration of the manure.<p>The results of experiments with cow manure give an insight in the stability of the process during several successive filling periods at process temperatures of 15 and 20°C When digesting cow manure (influent COD= 95-113 g/l) at 15°C in an AC-system with a 15% inoculation, a 'steady state' can not be reached within the filling time of 100 days. An extra digestion period of 40-50 days is necessary for a complete conversion of the accumulated fatty acids. The course of the following filling periods is more or less identical. A change of manure composition (95 ->113 g/l COD, 1.9 ->3.5 g/l NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N) results in an increase of the digestion time in order to degrade the accumulated fatty acids. The gas production rate however, remains almost the same.<p>The AC-digestion of cow manure (95 g COD/l) at 20°C has been conducted with inoculation percentages of 1, 7 and 13% at a filling period of 100 days. Even with the lowest inoculation the accumulated fatty acids are nearly completely converted during the filling time of 100 days. When an inoculation of 7 and 13% is applied, a considerable part of the filling period is in a 'steady state'.<p>Considering the better perspectives for practical application of AC-systems for pig manure instead of cow manure, more extended research was conducted with pig manure.<p>When applying digested cow manure (15°C-AC-sludge) as inoculation for digesting pig manure (74 g COD/l; 3.7 g NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N/l) at 15°C higher gas production rates are found than for digesting cow manure (113 g COD/l; 3.5 NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N/l) under the same process conditions. However also with the digestion of pig manure it is not possible to covert the accumulated fatty acids within the filling period, a post digestion of ± 50 days is required. Theoretically a minimal inoculation of 25% is necessary in order to convert the accumulated fatty acids within the filling period. With filling periods of 270 days a 10% inoculation will be sufficient. At a manure production of 1 m3/day reactor volumes of respectively 133 and 300 m <sup><font size="-1">3</font></SUP>should be installed.<p>When digesting more concentrated pig manure (118 g COD/l; 5.9 g NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N/l) in an AC-system at 15°C this results in an accumulation of propionic acid. Even after extreme long digesting periods (100+180 days post digestion) no degradation of propionic acid is shown. Probably the high concentration of NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N (5.9 g/l) in the manure is the cause of stagnation of the propionic acid breakdown.<p>An increase of process temperature from 15 to 30°C with a similar filling time, results in an increase of as well the volumetric as the specific gas production. Both the hydrolysis and the methane generation increase with a raise of temperature from 15 to 30°C.<p>In contrast with a process temperature of 15°C higher temperatures (20 and 30°C) induce hardly any difference in gas production and fatty acid course when the manure concentration is lowered from 118 to 74 g COD/l; The accumulated fatty acids are degraded into methane respectively in 140 and 120 days, at (20°C at 30°C) this is respectively 50 and 40 days, after which a steady state occurs. The specific growth rate of the methanogens as well for 20 as for 30°C does not differ with concentrated or diluted manure.<p>Results of the executed research show, that intensive stirring in the AC-systems with a high initial loading (e.g. a 15% inoculation and a 100 day filling period at a process temperature of 15°C) has a strong negative effect on the degradation of propionic acid and to a lesser extent on acetic acid. In an AC- system with a high initial activity (e.g. a 10% inoculation and a 100 days filling time at process temperatures of 20 and 30°C hardly or no negative effect of stirring was found. It should be concluded that intensive mixing is process technologically unfavorably. Only for reasons of management some stirring can be useful, e.g. when removing the digested manure or when heating is applied.<p><strong>TOXICITY</strong><p>In Chapter 6 the results are discussed of the research dealing with the effect of organic matter and the NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentration on the digestion of cow manure in CSTR-systems, at a process temperature of 30°C and a retention time of 10 days. The organic matter concentration of the manure, within a range of 2-7%, has no effect on the digestion. The NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-Nconcentration however has a very distinct effect, as well on the hydrolysis as on the methanogenesis. The effluent fatty acid concentration increases exponentially and the hydrolysis declines linear with NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentrations in the range of 1.2-4.9 g/l The relation between the percentage hydrolysis in the reactor (H <sub>R</sub> ) and the NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentration can be empirically described with the equation:<br/>H <sub>R</sub> = 22.8-4.16 [NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N].<p>When digesting manure with a high NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N concentration (4.9 g/l) at relative short retention times, two different equilibrium effluent fatty acid concentrations can establish. The lowest concentration can be achieved through a temporarily lowering of the loading. Similar results were found with starting up digesters at lower temperatures as described in Chapter 2 and probably can be attributed to the inhibitory effect of fatty acids.<p>Results of research on CSTR-systems show a clear negative effect of NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N, this is however not the case with AC-systems at 20 and 30°C<p><strong>APPLICABILITY OF AC- AND CSTR-SYSTEMS.</strong><p>The digestion of manure in CSTR-systems for gas production on farm-scale is applied in the Netherlands since 1979. Between 1979-1982 ± 25 installations have come into operation on pig and dairy farms. The development of the energy prices is the main factor causing no further growth of biogas installations after 1982.<p>The earning-capacity of biogas installations was apart from the drop of energy prices, negatively influenced by the considerable lower efficiencies of the installations (especially on piggeries) found in comparison with laboratory and semi-technical investigations. The cause of this is the occurrence of 'pre- digestion' in the manure storage. In fact, every farmer has got already a digester, however the process is not optimized and the gas is not collected and used. This finding gave the main rise to investigate the possibilities of combined storage and digestion. The application of an AC-system, in which the storage and digestion is combined, is in principle also much more suitable for manure digestion on farm-scale than the CSTR-system, for no extra reactor and/or effluent storage is required. Present developments on manure handling, viz., the required extension of manure storage and the requirement to cover the storage gas-tight for eliminating emission of NH3, make that the storage starts to resemble a digester.<p>The results presented in this thesis show clearly that an increase of the storage time can result in an important raise of the gas production, even at low temperatures 15°C Since it is known that CH <sub>4</sub> contributes by far more severe to the greenhouse effect than CO <sub>2</sub> (Goossensen & Meeuwissen, 1990), the biogas production during storage should be avoided or optimized and used in order to save fossil fuel. In the latter way a contribution can be made to fight the greenhouse effect.<p>The obtained results also show that avoiding methane production is very difficult. When a suitable bacterial population has established the methanogenesis will develop irrevocably, unless inhibitory substances are used. The latter is from an environmental point of view not recommendable. At a manure temperature of 20°C as it can occur in summer, 1% inoculation is enough to start the digestion process within the filling period. Since practice shows it is impossible to empty manure storages completely, approximately 10-15% of the manure in general remains, it can be concluded from the obtained results that with the extension of storage time to 5 months or longer also at temperatures of 15°C, considerable gas production will appear. At the test accommodation for piggery at Rosmalen a 700 m3 manure silo was isolated, covered gas-tight and equipped with a simple heating system. During two years research was done on the combination of storage and digestion of manure at ± 18°C. When it becomes compulsory to cover manure storages gas-tight, the combination of manure storage and digestion will certainly becomes an attractive alternative.<p>The use of an AC-system is in principle suitable when long term storage is compulsory. In the present situation in the Netherlands, where much organic manure is produced, a major part of this manure will have to be processed. When processing manure a maximal regain and re-use of valuable products and a minimal use of energy should be aimed. Anaerobic digestion as a part of such a system is to be seen as a method to remove and convert a major part of the solid and soluble organic fraction of the manure into methane. The methane produced can be used as energy for further process steps. When there is no need for longer storage if manure is processed on a large scale, the AC-system is less attractive than the CSTR-system.<p>By anaerobic digestion only a part of the organic compounds can be converted into methane. A further removal of solids can be accomplished by mechanical separation after or if possible before digestion. Mechanical separation of the 'fresh' slurry, into a solid and soluble fraction, has the advantage of a possible separate digestion of the fractions, the soluble fraction in a high loaded reactor, e.g. an UASB and the solid fraction in a low loaded reactor. In this way the total required reactor volume can be considerably reduced.<p>The soluble fraction contains the major part of the nitrogen, while the solid fraction contains most of the phosphor. The nitrogen in the soluble fraction is mainly present as NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N. An in principle very attractive system for removal of NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N (and phosphate) in the soluble fraction is the stripping/absorption process. A diagram of this process is given in Figure 1 (v. Velsen, 1985).<p><img src="/wda/abstracts/i1413_1.gif" height="379" width="600"/><p>It is incomprehensible that this system or other modified systems of this (Drese, 1988) are not yet tested on large scale.<p>From the point of view of appropriate use of resources it is necessary to regain and recycle the large amounts of nitrogen from the manure and not as proposed sometimes (Koster, 1990) to convert this into nitrogen gas by means of nitrification/denitrification, especially because this requires vast amounts of high grade energy. Complete recovery of NH<font size="-1"><sub>4</sub><sup>+</SUP></font>-N is not always necessary or economically justified. A combination of the stripping/absorption with the nitrification/denitrification process should at least be considered.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Lettinga, G., Promotor, External person
Award date3 May 1991
Place of PublicationS.l.
Publisher
Publication statusPublished - 1991

Keywords

  • sewage sludge
  • digestion
  • septic tanks
  • manures
  • manure surpluses

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