Bonmatı´ and Flotats
Table 2. Characteristics (average of 3 repetitions) of the concentrate in batch evaporation tests (standard deviations are
the evaporator wall. The incom-
plete total NH3-N condensation
should also be taken into ac-
count as another possible source
of error.
indicated within brackets).
Fresh Slurry Concentrate
5
Digested Slurry Concentrate
5
Initial
pH
6
4
6
4
pH
6.3 (Ϯ0.04)
230 (Ϯ3.70)
22.3 (Ϯ0.60)
4.8 (Ϯ0.37)
27.5 (Ϯ1.43)
5.3 (Ϯ0.04)
214 (Ϯ63.7)
21.0 (Ϯ5.76)
4.5 (Ϯ1.29)
24.3 (Ϯ8.20)
4.6 (Ϯ0.13)
309 (Ϯ101)
30.9 (Ϯ8.29)
6.9 (Ϯ2.63)
37.4 (Ϯ9.78)
6.5 (Ϯ0.06)
311 (Ϯ1.92)
41.2 (Ϯ4.84)
6.8 (Ϯ0.40)
78.5 (Ϯ7.46)
6.2 (Ϯ0.03)
214 (Ϯ3.98)
28.6 (Ϯ4.64)
5.3 (Ϯ1.00)
53.8 (Ϯ6.92)
5.9 (Ϯ0.11)
262 (Ϯ9.69)
36.6 (Ϯ8.51)
6.3 (Ϯ2.96)
66.9 (Ϯ11.77)
Semi-Continuous
Evaporation Tests
Evaporation Rates. Because the
final TS concentration in the
TS (g/kg)
TKN (g/kg)
P (g/kg)
K (g/kg)
semi-continuous
evaporation
experiments was between 200
increase in batch evaporation experiments with landfill
leachate, concluding that a continuous addition of acid
was needed to maintain the desired pH. This pH in-
crease was higher in the experiments performed with
digested slurry than in those with fresh slurry. This
showed that when using a given initial slurry pH, the
NH3-N and un-ionized VFA fraction of the two slurry
types were different, explaining the differences ob-
served in the percentages of the compounds trans-
ferred.
and 250 g/kg, it can be assumed that the water removed
was the free-water fraction, and a constant evaporation
rate could therefore be expected. The different character-
istics of the slurries tested did not have an important
effect on evaporation rates. In the steady-state period, the
evaporation rates reported in the fresh slurry experiments
were 45.9 mL condensate/hr, while in the digested slurry
experiments it was 43.0 mL condensate/hr.
Condensate characterization. The evolution of condensate
characteristics (NH3-NT, TVFA, COD, and pH) throughout
the semi-continuous experiment are shown in Figure 7.
As seen, the studied parameters behaved differently in the
two periods. During the transitory period, there were high
variations in the composition of the condensate. COD,
total NH3-N and TVFA concentration increased linearly
with the degree of concentration of the slurry. The lowest
concentrations were reported after each new addition of
slurry, and maximum values were reported just before a
new slurry addition (Figure 7). This showed that the de-
gree of slurry concentration also had a major effect
on condensate characteristics.8 The almost constant con-
densate characteristics during the steady-state period in-
dicated that a stable running of the system had been
achieved (Figure 7).
In all cases, COD % transferred to the condensate in
experiments with fresh slurry was significantly higher
than the values obtained with digested slurry (Figure
6c). The different natures of the organic matter of
the slurries may explain this fact. Because most of the
volatile organic matter is consumed during anaerobic
digestion, low biodegradable or non-volatile organic
matter mainly composes the COD of the digested
slurry.
Concentrate Characterization. One of the objectives of the
evaporation treatment is to obtain a concentrate with a
higher nutrient concentration than that in the original
slurry. The main characteristics of the concentrate are
shown in Table 2. Because P and K are non-volatile
compounds, the resulting concentrate should contain
the entire initial P and K present in the non-treated
slurry. The mass balance performed by applying eq 3
indicated that between 84 and 95% of the initial P and
between 90 and 97% of the initial K were retained in
the concentrate. The missing P and K could be attrib-
uted to experimental error.
Steady-state condensate characteristics were com-
pared with those from the batch experiments, with the
initial slurry pH equal to 5. As expected, the higher evap-
oration temperature in the semi-continuous experiment
(55 °C as opposed to 40 °C in the batch experiments) lead
to higher condensate concentrations of NH3-N, TVFA,
and COD in the semi-continuous (Table 3) than in the
batch experiments (Figure 5). Similarly, NH3-NT, COD,
and TVFA percent transferred to the condensate were
larger in the semi-continuous than in the batch experi-
ments (Figure 6).
Between 83 and 84% of the initial TKN was retained
in the concentrate in experiments with fresh slurry and
between 80 and 84% was retained in those with digested
slurry experiments. No relationship was found between
the initial slurry pH and the percent of TKN retained, and
no differences were found between the two slurry types.
This could be explained by both the small final volume of
the concentrate and the observed scaling phenomena in
Comparing the two slurry types, as in batch exper-
iments, VFA % and COD % transferred to the conden-
sate were larger for fresh slurry semi-continuous exper-
iments than those for digested slurry. However, NH3-NT
% transferred showed similar values for the two slurry
28 Journal of the Air & Waste Management Association
Volume 53 January 2003