FR2477522A1 - Treating manures by two stage anaerobic fermentation - followed by culturing in ponds with algae - Google Patents

Abstract

Two-step process for complete purification treatment of organic-rich manures comprises first anaerobic fermentation in two stages with first (acidification) stage at 30-42 deg.C and pH 4.5-6.5 for 5-20 (12-20) days and second (methanation) stage at 48-60 deg.C and pH 6.8-8.3 (7.7-8.3) for 5-20 (5-10) days. In the second stage the diluted effluent its cultured in ponds with algae at ambient temp. for 3-10 days, and the final mixt. is then decanted to give a supernatant suitable for direct discharge. Pref. the effluent from the first step is diluted to a COD below 2g per l and the algal fermentation is carried out using unicellular green algae of which 80% are of the genus Chlorella. The method is esp. useful for treating effluent from intensive pig raising units. By using two anaerobic stages each optimised for a particular type of bacteria, the overall quality of purification is improved. Algae harvested from the second step can be used as fertiliser; animal feed (or esp. feed additive) or as feedstock for the first-step fermenters. The methane generated can be used as fuel for running the process.

Description

 The subject of the present invention is a process for the complete treatment of purification of effluents rich in organic matter, comprising in a first step an anaerobic fermentation and in a second step an algae culture.

 It is well known to subject effluents containing organic materials to anaerobic fermentations especially to produce methane. This is what is called methane fermentation.

 It is also known to purify liquid effluents by submitting them in lagoons to algal cultures. However, the use of this process is limited both by the composition of the effluents that are not all suitable for good purification and by the size of the lagoons that it is necessary to provide to obtain a sufficient purification efficiency.

 The subject of the present invention is a process which makes it possible, in the case of a particular effluent, the effluent resulting from an intensive breeding of pigs that will be hereinafter referred to as "slurry", both to produce a large quantity of methane, source of energy and to obtain a complete purification of the manure allowing the rejection to the nature.

 Pig manure consists essentially of urine, faeces and food residues of animals, to which wash water is often added. It is in the form of a thick liquid effluent, heavily loaded with organic matter.

 Intensive livestock waste, slurry type, is an effluent containing from 1 to 5% organic matter, most of which is biodegradable anaerobically.

 Anaerobic fermentation can be carried out either in a single phase or in two phases. In the latter case, the first phase in which organic materials, complex molecules, in particular polysaccharides, protein and lipidic cellulose compounds, are progressively transformed into short-chain organic acids and alcohols is generally referred to as the acidification phase, and the second phase in which these compounds are transformed by fermentation into methane is called the methane phase.

 The implementation of this fermentation poses a certain number of problems related to the different kinetics of transformation of the products of the medium and to the different activities of the various bacteria present in the medium, notably as a function of the physicochemical conditions, temperature, pH, etc. fermentation medium.

 The aim of the invention is a process which makes it possible both to optimize the conditions of the anaerobic fermentation and thus to obtain significant yields of methane and to make it possible, thanks to the complementarity of the two stages, to completely purify the particular effluent. treaty.

 The subject of the invention is a process for the complete treatment, of purification in two stages, of effluents rich in organic matter of slurry type with simultaneous production of methane, characterized in that during a first stage a fermentation is carried out. anaerobic two-stage, the first stage of the fermentation, called acidification, being carried out in a reactor at a temperature between 30 and 420 C, in a pH range between 4.5 and 6.5 and with a residence time of the effluent between 5 and 20 days, the second stage of the fermentation, called methane, being carried out in a reactor at a temperature of between 48 and 600 ° C., in a pH range of between 6.8 and 8.3, and with a residence time of between 5 and 20 days and then during a second stage, after diluting the effluent from the first stage, it is indroduced in a lagoon of a depth generally between 10 and 30 cm where an algae culture is carried out at room temperature with a residence time of 3 to 10 days.

 Finally, at the end of this second stage, a separation by decantation is carried out, allowing the supernatant to be rejected in the natural environment.

The various strict and facultative anaerobic bacteria are present in the fermentation medium. For example, we can cite
methanobacterium suboxydans
methanobacterium propionicum
methanosarcina methanica
methanobacterium omelianskü
methanobacterium formicium
methanosarcina barkerü
etc.

 These bacteria have various activities and some actively participate in the first phase of fermentation, acidification, and others actively in the second phase, methane fermentation.

 It was surprisingly found that the physicochemical operating conditions of these two phases of the fermentation had a great importance on the optimization of this first stage, and on the quality of the effluent sent to the second stage, allowing a net overall improvement in the quality of purification.

It is indeed important that the first phase be implemented in the so-called mesophilic temperature zone (300 C <t <420 C) in order to favor the activity of bacteria allowing the transformation of organic matter into acids, alcohols and aldehydes. short chains, at the expense of the methanic transformation, and that the residence time of the effluent in the reactor during this phase is preferably high 12 to 20
days, compared to the residence time during the methane phase.

 the second phase is carried out in the so-called thermophilic temperature zone (480 C <t <600 C) in order to favor the activity of the bacteria allowing the methanic transformation and that the residence time of the effluent in the reactor during this phase is preferably low 5 to 10 days compared with the residence time during the acidification phase.

 The two phases are carried out under substantially different conditions of p1.5 to 6.5 for the acidification phase 6.8 to 8.3 for the methane phase, and preferably between 7.7 and 8.3 for this second phase.

 For the realization of this first step will be provided in a fermentation medium intermittent and gentle agitation. Any suitable means of agitation may be used for this purpose; for example a recirculation of the gases produced by the first-phase reactor ensures sufficient homogenization of this reactor.

 The effluent from the first stage still contains a significant organic load representing 50% of the initial charge.

This effluent is then diluted so that its COD (Chemical Oxygen Demand) is less than 2,000 mg / l and directed to a lagoon with a depth of between 10 and 30 cm in which algae develop. These unicellular green algae generally comprise 80; Chlorophyceae of the chlorella genus. The residence time of the effluent diluted in the lagoon is generally between 3 and 10 days and the temperature of the medium is preferably between 18 and 300 C.

 It should be noted that the prior anaerobic fermentation of the effluent is essential for the proper operation of the lagoon. In fact, the suspended solids content of raw manure is too great to allow the installation of photosynthetic micro-organisms in the lagoon.

At the exit of the lagoon, at the end of the second stage, the effluent is separated by decantation. The supernatant, purified, can then be rejected in the natural environment. In fact, a portion is generally used to dilute, at the inlet, the effluent from the first stage. The algae recovered by settling out of the lagoon can be used in different ways. as fertilizer,. as a feeding element for livestock, by incorporation,
after drying, at feeding rations,. as a filler, first-stage fermentors, in a mixture
with slurry, to be bioconverted to methane.

Example
The purification process of the invention was carried out on a pig slurry having the following characteristics:
. Total nitrogen 4.200 mg / l
. Ammonia nitrogen 2,800 mg / l
. COD 16,500 mg / l
. Dry matter 15 g / l
of which organic matter 7.5 g / l
. pH 7.7
The operating conditions of the process steps were as follows - Step One
Acidification Methanation
Volume (1) 100 50
T (OC) 37 50
pH 6.3 8.1
Time to stay (d) 14 7
Discontinuous Mechanical Agitation Discontinuous Mechanics - Second Stage
Residence time: 7 days
Temperature: 17 to 250 C
daytime pH: 8.7
nightlife: 7.8
The following results were obtained
Reactor outlet Reactor outlet
acidification of anaerobic digestion
Total nitrogen (mg / l) 3,200 2,500
Ammonia nitrogen (mg / l) 2,200 2,100
COD (mg / l) 15,000 8,500
Dry matter (g / l) 13 12 including organic matter (g / l) 6.5 3.6 pH 6.3 8.1
The total gas production is 950 kg of organic matter and the composition of the gas is in volume
. Methane: 63%
. Carbon dioxide: 23%
. Nitrogen: 14 ob
The effluent from the methanation reactor is diluted ten times and introduced into the lagoon where the algae develop.

The characteristics of the input effluent and the liquid output phase are as follows
Entry Exit% purge
supernatant
COD mg / l 850 77 91
Nitrogen mg / l total 250 35 86.5
Phosphorus mg / l total 224 120 46.5
In the first stage, the gas, that is essentially a mixture of methane and carbon dioxide, is produced mainly during the second phase and secondarily during the first phase. This mixture can be used as such as fuel. For example, one fraction of the gas may be burned to heat another fraction of the gas and reintroduce this heated gas into the fermenter to ensure both heating and agitation of the fermenter.

 It is also possible to separate the constituents of the mixture, methane and carbon dioxide, to obtain on the one hand pure methane and on the other hand carbon dioxide that can be recycled into the methane reactor to produce an additional methane.

Claims (5)

1 - Complete treatment process, in two stages, of purification of ef  fluents rich in organic matter slurry type, with  simultaneous production of methane characterized in that, during  in a first step an anaerobic fermentation is carried out at two  stages, the first stage of fermentation, called acidification,  being implemented in a reactor at a temperature of  between 30 and 420 C in a pH range of 4.5 to 6.5  and with a residence time of the effluent of between 5 and  20 days, the second stage of the fermentation, says methane,  being implemented in a reactor at a temperature of  between 48 and 600 ° C, in a pH range between 6.8 and 8.3  and with a residence time between 5 and 20 days, then at  during a second stage, after diluting the effluent from  the first step, we introduce it in a lagoon where we realize  a culture of algae at room temperature with a time of  stay of the effluent from 3 to 10 days, finally, at the end of this  second stage a separation by decantation, per  putting the rejection of the supernatant in the natural environment.  2 - Process according to claim 1 characterized in that during the  first step the residence time of the effluent in the reactor  the first floor is between 12 and 20 days and that time  of residence of the effluent in the second stage reactor is  between 5 and 10 days. 3 - Process according to claim 1 characterized in that the second  phase of the first stage is carried out at a pH between  7.7 and 8.3. 4 - Process according to claim 1 characterized in that the diluate  the effluent from the first stage is such that  the effluent introduced in the second stage has a COD lower than  2,000 mg / l. 5 - Process according to one of claims 1 and 4 characterized in that  that the culture of the second stage is carried out with the algae  unicellular green cells usually containing 80% chlorophycea  of the chlorella genus.