Changes in Particle Size Distribution in Hydroponic Lagoons Working as A Third Stage of Wastewater Treatment

The aim of research was to evaluate changes in the particle size distribution in two hydroponic lagoons working as a third stage of wastewater treatment where purication processes are based on plants and algae activity. Wastewater samples were taken during the winter and summer half of the year. In the samples taken from analyzed hydroponic lagoons the range of particles size was very wide (0.01 – 1000.0 μm). In both of the research facilities, the fractal dimension (Df) of particles was close to 2.0 what decides about developed surface of the particles. The results may indicate the predominance of algae cells in the total amount of suspensions. This information may pay a key role in determination the impact of discharged particles on the receiving water bodies quality as well as can be the main factor that allows to improve the system of suspended solids removal.


Introduction
Due to high risk of eutrophication in surface water reservoirs -natural ponds, lakes, arti cial reservoirs and even seas -limiting the in ow of nitrogen and phosphorus compounds should be currently the priority 1,2 . Among the sources of nutrients getting into surface waters we can distinguish linear, surface and point sources. Linear sources are mainly roads whose highly sealed surface generates surface runoff washing contaminants into the trenches and then to surface water 3 . The largest percentage of pollutants owing down the roads are suspensions carrying a nitrogen and phosphorus load as well as the nitrogen compounds from fertilizers derived from arable lands nearby the roads 4,5 . The main surface sources of nutrients are agricultural lands where the use of mineral and organic fertilizers rich in N and P is a common action. The threat of watercourses and surface water enrichment in nutrients is high especially during early spring when fertilizers are used 6 . Point sources of nitrogen and phosphorus that enter surface waters include, above all, sewage discharges from wastewater treatment plants (WWTP) 7 . This source can be called one of the most important because the sewage ows into the receiver in a constant way and the amount of nutrients is subject to periodic analysis. What more the e ciency of N and P removal in WWTP is still increasing by the use of speci c solutions.
One of the solutions invented for increased nitrogen and phosphorus removal is the use of additional wastewater puri cation after biological treatment. The common solution is the hydroponic system where aquatic plants are owing on the wastewater surface planted on oating medium. This kind of oating islands ( oating hydroponic root mats FHRMs) are successfully used for improvement of water quality in lakes or other reservoirs 8 . To remove excessive amount of N and P from sewage, a hydroponic ditch with aquatic plants oating on a oating islands can be used. The plants roots immersed into wastewater work as a lter for owing sewage and provide suitable hydraulic conditions for contaminants uptake 9 .
Additional treatment (3rd stage of wastewater treatment) with the use of arti cial river is based on natural processes occurring in natural watercourses called self-puri cation. Reduction of nutrients and other contaminants concentrations in semi-natural puri cation systems is a result of plants, bacteria and other aquatic organisms activity 10,11,12 . Very often the role of algae in this kind of systems is omitted while their nutrient uptake capacity is very high 13 . As reported by Orfanos et al. 14 algae are able to remove up to 97% of total phosphorus, 93% of phosphates and 99% nitrate nitrogen from secondary e uent from different kinds of industries.
Nowadays the algae based wastewater treatment methods are becoming more popular due to the complex puri cation process which consists of nutrient uptake and retention in algae cells with simultaneous ammonia stripping and oxygen enrichment 15,16 . At the same time high production on biomass and possibilities of its use for biofuel production is admittedly an additional advantage 17 .
Unfortunately the use of natural organisms in technology for 3rd stage of wastewater treatment may increase concentration of total suspended solids at the out ow from WWTP. Dead plant fragments, dead aquatic organisms and excess algae biomass that is not used for further purposes, may constitute a large percentage of suspended solids discharged into the receiver causing a threat for natural water ecosystem. The most problematic is removal of excess amount of algae because of their small size, negatively charged surface, small density and lipids storage in cells which make them permanently suspended in sewage 18 . Fast and easy method of total suspended solids characterization might be useful in development of new, effective methods of suspensions removal and reuse. Such method is laser granulometry that allows to measure particles size and dimensions, their distribution in wastewater as well as their shape and speci c properties, e.g. the ability to react with other wastewater components 19,20 . Information about the size of suspended solids as well as their fractal dimensions

Samples collection and measurement
Wastewater samples were taken during the winter and summer half of the year from the several sampling points of hydroponic ditch -always from the in ow of biologically puri ed sewage, the middle points of the hydroponic lagoon and the out ow of treated wastewater (Figs. 3 and 4). In WWTP A the P1 point represents sewage clari ed in secondary settling tank, P3 -sewage after ow through part of the ditch planted with macrophytes and two aerated sections, P4 -sewage after ow through the middle part of the lagoon without plants but with aerated sections and P6 -sewage after ow through planted section at the out ow of the lagoon. Last 10 m of the last section of the lagoon are not equipped with oating islands. In WWTP B the L1 point represents sewage after clari cation in secondary settling tank, L2after ow through the part of a ditch planted with Calla pallustris and the L3 -sewage at the out ow of the lagoon, after ow through the part of the lagoon without oating panels. In both objects, samples were collected from the same depth and with the same conditions of sewage ow through the lagoon.
After samples collection, 800 ml of the wastewater sample was transported to the laboratory to test the granulometric composition of suspended solids in sewage with the use of the Malvern Mastersizer 2000 laser granulometer. To provide homogeneous probe for laser diffraction measurements, the Hydro MU dispersion paddle with stirrer was used. Each time, 30 measurements for one sample were made to get a representative number of results for analysis. The calculations of equivalent diameters and fractal dimensions were performed using an Excel spreadsheet prepared by Malvern Instruments Ltd., with a restricted computational procedure.

Results And Discussion
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 ow through the hydroponic ditch in the winter season, were not very noticeable especially in the rst sections of the lagoon. It can be noticed that characteristics of particles diameters of two rst 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 ow through the part of the ditch without plants. The particles with di in a range 0.1-1.0 µm are classi ed as microsuspensions that do not undergo sedimentation 21 . 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 rst sampling points was observed -in L1 and L2 points 90% of particles had diameters of 10 µm. At the out ow 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 identi ed 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 in ow to the lagoon 90% of particles had di = 10.0 µm, while in the out ow 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 out ow of the lagoon were smaller than from WWTP B.  Table 1 -for WWTP A and Table 2 -WWTP B.  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 ow through the rst two sections of the lagoon and increased in the nal section giving higher values at the out ow than at the in ow. Although the D(4. 3) values at the out ow were higher than at the in ow, 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 in ow and the out ow 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 ow through the last section of the hydroponic ditch but the nal values were similar to the ones at the in ow. 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 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 identi ed in hydroponic lagoons during winter season are presented in Fig. 7 (A, B). At the out ow 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.

Conclusions
The analyses of granulometric composition of wastewater owing through the hydroponic lagoons in two different wastewater treatment plants (WTTP A and WWTP B) in the summer and winter half of the year showed a lot of similarities. The changes observed between particles size and equivalent diameters from two objects might be connected with the differences of hydroponic ditch construction (length, depth, used plants). In the object with shorter hydroponic ditch the differences between amount and size of particles were more noticeable than in the longer one. In both of the objects the slight decrease of fractal dimension of particles was observed but still was close to 2.0. The Df of particles close to 2.0 gives an information that the particles shape is not linear but have more developed surface. The particles with linear shape were identi ed only in the WWTP B in the summer season, where Df decreased from 2.149 to 1.089 after wastewater ow through the lagoon.
Information about particles size, they reactivity and sedimentation capacity may pay a key role in determination the impact of discharged particles on the receiving water bodies quality as well as can be the main factor that allows to improve the system of suspended solids removal. In semi natural sewage puri cation systems like hydroponic lagoons particles characteristics is even more important because of the growth of algae that discharged into the receiving watercourses might be harmful for inhabitants of natural ecosystems. Comparing the data obtained in granulometric composition measurements with identi cation of algae species is the object of further Authors research.