Calculations of particle size distribution showed that in all samples taken from hydroponic lagoons the range of particles size was very wide (0.01–1000.0 µm). In WWTP A the changes of particles diameters during the flow through the hydroponic ditch in the winter season, were not very noticeable especially in the first sections of the lagoon. It can be noticed that characteristics of particles diameters of two first sampling points – P1 and P3 – are nearly the same and 90% of particles had di = 1.0. Very noticeable increase in the share of particles with diameters lower than 1.0 occurred in the next sampling point – P4 – after flow through the part of the ditch without plants. The particles with di in a range 0.1–1.0 µm are classified as microsuspensions that do not undergo sedimentation21. At the last sampling point (P6) the diameters increased and 90% reached sizes bigger than 5.0 µm (Fig. 5.A). In the WWTP B similar tendency in two first sampling points was observed – in L1 and L2 points 90% of particles had diameters of 10 µm. At the outflow of the lagoon the di of particles decreased and the vast majority showed diameters smaller than 1.0 µm (Fig. 5.B).
In the summer season particles diameters identified in hydroponic lagoon of WWTP A showed less variability than in the winter season (Fig. 6.A). The vast majority of particles in sewage samples from all sampling points had di in a range 1.0–10.0 µm. It can be observed that there was a slight increase in particles diameters at the end sampling points. In the second research object (B) the constant increase of particles diameter was observed. At the inflow to the lagoon 90% of particles had di = 10.0 µm, while in the outflow from the wastewater treatment plant (L3) diameters of 90% of particles reached 100 µm (Fig. 6.B). Comparing the particles diameters from both of research object it can be concluded that in the hydroponic lagoon of WWTP A the particles diameters at the outflow of the lagoon were smaller than from WWTP B.
To determine the characteristics and structure of the suspended particles in sewage from hydroponic lagoons, the average equivalent diameters (Di) were calculated. The D(1.0) describes the average length of particles, the D(2.0) – average surface, D(3.0) average volume22. The D(3.2) equivalent diameter is calculated on the basis of the volume ratio referred to the sum of particles surface and its value decides about the size of active surface of the particle. The bigger the D(3.2), the surface of particles is smaller and they efficiency of chemical reactions catalysis decrease23. The size of D(4.3) diameter informs about the location of gravity center in the particle and mass concentration of the particles in the solution. The value of D(4.3) decides about sedimentation capacity of the particles24. The results of equivalent diameters calculations are presented in Table 1 – for WWTP A and Table 2 – WWTP B.
Table 1
Equivalent diameters of particles identified in wastewater from hydroponic lagoon in WWTP A
Winter season
|
|
D(1.0)
|
D(2.0)
|
D(3.0)
|
D(3.2)
|
D(4.3)
|
P1
|
0.923
|
1.587
|
3.239
|
13.494
|
27.918
|
P3
|
0.875
|
1.441
|
2.952
|
12.392
|
27.714
|
P4
|
0.160
|
0.188
|
0.281
|
0.624
|
2.544
|
P6
|
2.782
|
4.246
|
7.502
|
23.418
|
53.313
|
Summer season
|
P1
|
3.141
|
6.02
|
11.208
|
38.851
|
58.008
|
P3
|
2.698
|
4.011
|
8.139
|
33.517
|
76.443
|
P4
|
4.580
|
7.316
|
13.313
|
44.085
|
87.062
|
P6
|
4.232
|
6.480
|
10.849
|
30.414
|
57.508
|
Table 2
Equivalent diameters of particles identified in wastewater from hydroponic lagoon in WWTP B
Winter season
|
|
D(1.0)
|
D(2.0)
|
D(3.0)
|
D(3.2)
|
D(4.3)
|
L1
|
4.180
|
7.084
|
13.440
|
48.377
|
88.165
|
L2
|
4.368
|
7.220
|
13.203
|
44.152
|
78.340
|
L3
|
0.325
|
0.410
|
0.704
|
2.080
|
6.896
|
Summer season
|
L1
|
5.909
|
9.268
|
15.839
|
46.264
|
87.049
|
L2
|
21.966
|
27.907
|
41.586
|
92.343
|
213.839
|
L3
|
54.865
|
62.004
|
73.682
|
104.051
|
167.563
|
In the winter season in hydroponic lagoon of research object A, the identified D(1.0) diameters varied from 0.160 to 2.782 µm. The average length of particles from P1 to P4 sampling points was decreasing and then increased to almost 3.0 µm at the outflow. D(2.0) equivalent diameter reached the highest value at the outflow from the lagoon when it reached 4.246 µm. During the flow through the ditch also D(3.0) diameters changed – at the inflow D(3.0) = 3.239 when at the outflow – 7.502 µm. D(3.2) also increased during the flow through the ditch from 13.494 µm to 23.418 µm. This change indicates that the reactivity of the particles at the outflow of the lagoon was smaller than reactivity of particles in biologically treated wastewater. Similar tendency of Di changes was observed in the case of all calculated equivalent diameters (also the D(4.3)) – the diameters decreased during the flow through the first two sections of the lagoon and increased in the final section giving higher values at the outflow than at the inflow. Although the D(4.3) values at the outflow were higher than at the inflow, they still suggested that the sedimentation capacity of the particles was low (D(4.3) < 50 µm).
During the summer season, the changes between equivalent diameters at the inflow and the outflow were almost imperceptible. They were slightly changing the P3 and P4 measuring point, when the decrease was noticeable. As in the winter season – the Di sizes increased during the flow through the last section of the hydroponic ditch but the final values were similar to the ones at the inflow. It can be noticed that during the warmer period, the Di were higher than during the winter season. The reason might be connected with higher temperatures of wastewater what is the main factor affecting activity of microorganisms and algae.
In WWTP B the average length of identified particles in sewage from hydroponic lagoon expressed by D(1.0) varied from 4.180 µm at the inflow to 0.325 µm at the outflow. The average surface described by D(2.0) also decreased during the sewage flow in the lagoon from 7.084 µm to 0.410 µm. In both described equivalent diameters, after flow through the first section of the ditch, slight increase of the Di values was observed. The average volume of particles changed from D(3.0) = 13.440 to 0.704 µm but as well as in the case of D(3.2) and D(4.3) after the first section of lagoon there was a slight decrease in value and then after passing through the next stage increased by up to 95%. Very small D(3.2) value at the outflow – 2.080 µm as well as very small D(4.3) = 6.896 µm indicates, that particles in sewage flowing out of the wastewater treatment plant have bigger reactivity than at the inflow but still their sedimentation properties are bad.
During the summer season the tendency observed during the winter changed. In all cases
a very high increase in the value of equivalent diameters during the purification in 3rd stage of sewage treatment was observed. The average length, surface and volume of particles increased several dozen times what also affected sizes of D(3.2) and D(4.3) diameters. The efficiency of chemical reaction catalysis of the particles decreased because of the relatively high D(3.2) value that reached 104.051 µm. A positive aspect of the changes occurring in the hydroponic ditch is the increase in the diameter D(4.3) and the improvement of the sedimentation capacity of the particles.
Average fractal dimensions of particles in wastewater from hydroponic lagoons were calculated to determine the spatial distribution of particles. According to Valle et al.25 the shape of particles with Df close to 1.0 has linear character, Df ≈ 2.0 is assigned to particles with more developed surface when the highest values that reach 3.0 are characteristic for particles more concentrated around the nucleus, with more extensive surface. The average fractal dimensions of particles identified in hydroponic lagoons during winter season are presented in Fig. 7 (A, B).
Analyses of average fractal dimensions of particles in sewage from hydroponic ditch in WWTP A show that in all sampling points the Df was close to 2.0 with coefficient of determination R2 on the level of more than 0.99. The values decrease during the 3rd stage of treatment but because of very small difference on the level of 0.063 (with the R2 = 0.5323) the changes of particles shape are not significant. In the case of fractal dimensions of particles in wastewater from WWTP B the tendency is similar – Df decrease during the treatment. Changes in fractal dimensions between inlet and outlet of the lagoon are more significant (R2 = 0.7475) but the particles shape does not change diametrically. The highest Df reached 2.080 and the lowest one – 1.778. To compare characteristics of particles identified in wastewater during winter and summer time, the Df of particles measured in sewage samples taken during the summer season are presented in Fig. 8.
In the research object A the average fractal dimensions of particles at the inflow and the outflow of the last stage of sewage purification were nearly the same and reached 2.330 (inflow) and 2.161 (outflow). Decrease of Df value was observed in the second sampling point – P3 where it reached 1.859. For all calculated fractal dimensions the coefficient of determination R2 was higher than 0.99 what indicates very good match and hence - high accuracy of calculations26. In the case of WWTP B the decrease of fractal dimension after treatment in hydroponic lagoon was clearly noticeable – the Df decreased from 2.149 to 1.089 what indicates changes in particles formation. At the outflow from the hydroponic ditch particles of suspended solids had linear shape as opposed to particles in biologically treated wastewater, whose surface was more extensive in space.