Chapter 11
DNAPL
CONSTRUCTED WETLAND FOR WATER TREATMENT
Case Study-Treating Wastewater From Dairy Parlors
During the 1970s-80s in Italy, among constructed wetland systems used in the livestock production sector, attention was focussed primarily on surface flow systems, in which floating macrophytes such as duckweed (Lemna spp.) or water hyacinth (Eichhornia crassipes) were grown over broad surfaces submerged by wastewater, also those particularly rich in organic substance and nutrients The hope was to accumulate energy and large quantities of nutrients in the plants, whose biomass could be profitably reused for energy purposes (methane fermentation) or for animal feed;unfortunately this was not possible due to operational difficulties and unsustainable costs.Over the last ten years, there has been increasing use of subsurface flow systems for treatment of municipal wastewater of small villages that are far from the sewage mains (These systems consist of wetlands impermeabilised and filled with inert material of different particle size (crushed stone, gravel, sand), on which plants are grown, such as reed (Phragmites), cattail (Typha), and rush (Scirpus).This type of system can be constructed so that the wastewater to be treated runs through the filling substrate in a direction that is prevalently horizontal (SFS-h) or prevalently vertical (SFS-v). While in the former case the movement of the wastewater is more or less continuous, in vertical flow wetlands it is intermittent, and therefore the bed can be periodically re-oxygenated, with important implications on the purification processes that take place there.
On livestock farms, as other experiences have demonstrated, the application of subsurface flow systems can only be conceived for wastewater with a lower load of organic matter and nutrients, similar to municipal wastewater, as are those deriving from washings of the milking areas not trod on by the cows In the dairy sector, loose housing barn with the milking operation in a dedicated parlour has become more and more common. This operation gives rise to considerable volumes of wastewater with a low content of fertilising elements, for which storage and spreading along with other livestock effluents are problematic and uneconomic. For this type of wastewater, it could be more appropriate the use of purification treatments which, in respect to the laws parameters, enable wastewater re-use or disposal in the sewage systems or in surface waters. Due to its low environmental impact, reduced or nil energy consumption, and simplicity of operation, the technique of horizontal subsurface flow constructed wetland may constitute an interesting solution While in the tie-stall barn the quantity of wastewater produced during milking operations normally does not exceed 10-15 L/day per lactating cow (washing of the pipes, milking units, and milk room), in loose housing barn the production of wastewater is generally higher. In fact, in addition to the washing water of the milking system and the milk room, wastewaters deriving from the holding area and the milking parlour must be considered.Optimising the sizing of the various areas as well as the washing operations and water consumption, it would be reasonable to estimate wastewater production in the milking center of 50-55 L/day per cow under production, which tends to decrease when the herd size increases (Rossi and Betti, 1999).
Methods
A demonstration horizontal subsurface flow constructed wetland was set up at the Santa Lucia farm in Casina (682 m a.s.l., province of Reggio Emilia, Italy) in 1999. The wetland treats the wastewater coming from the washing of the milking pit, stalls, milking system, and bulk milk tank of a milking center with a herring-bone parlour with 5+5stalls. The system was sized considering the presence of 80 lactating cows and the opportunity to include in the treatment the domestic sewage coming from farm inhabitants.
Plant Description. The plant (is essentially composed of: (1) a well for inspection of the inflowing wastewater to the plant; (2) an Imhoff type septic tank and a plastic filter for the removal of sedimentable suspended solids; (3) two SFS-h constructed wetlands (reed beds) parallel to each other, sizing 12 x 6 x 1 m each, and planted with reed (Phragmites australis); (4) wells at the outflow of the reed beds; (5) a system for recirculation of the waters coming out of the wetlands.Immediately upstream from the area of the reed beds, a drainage channel filled with gravel was built to collect the rainwater run-off that could flow into the two wetlands from the surrounding areas. The bottom of the two reed beds was suitably impermeabilised using a synthetic covering in PVC protected on both ends with geotextile. One wetland was filled with washed pea gravel of 3-6 mm diameter, and the other with washed gravel of 8-12 mm diameter. Near the wastewater inflow and outflow points of both beds a layer of coarse gravel (diameter 8-35mm) was put to favour the flow of water at the bottom of each section. Perforated tubes for inspecting the water flow that crosses each of the two beds were placed vertically at 1/4, halfway, and 3/4 of the length of each wetland. The homogeneous distribution of the wastewater at the head of each wetland is guaranteed by special channels whose discharge points onto the gravel beds can be regulated separately. The height of the wastewater outflow pipes in the wells at the outflow of the reed beds can be regulated; also the water level inside each reed bed can be controlled. The recirculation system includes a centrifugal pump with a float that makes it possible to redirect the outgoing wastewater from one wetland to another through a plastic pipe.
A minimal colonisation by weeds (Solanum nigrum, Taraxacum officinale, Rumex conglomeratus, Populus nigra, Salix caprea, Anagallis arvensis) occurred, and no intervention of any type was necessary for controlling them.
Results and discussion
The average values and the range of fluctuation for each of the parameters measured at the inflow and outflow of the plant (sampling points 1 and 5), the average percentage reductions found and, for purposes of comparison, the limits of the current regional laws of the Emilia-Romagna Region for discharging these wastewater into the surface water (Table III Regional Law no. 7 of 29 January 1983).
Although the organic and nutrient load of the wastewater flowing into the plant are higher than those of the typical municipal wastewater, the percentages of removal obtained with the whole system are high, allowing to respect the regional limits imposed by the legislation.
For the first reed bed, the cleaning efficiency was greater than for the second bed.
The total reduction of suspended solids and organic load was consistently maintained at levels above 90%; those of the nutrients nitrogen and phosphorous were over 40% and 50%, respectively.
The comparison between the first period of operation, spring-summer, and the second, autumn-winter, made it possible to verify the increase in purification capacities of the second reed bed, probably due to the progressive plant and microbial colonisation, which occurred with a certain delay with respect to the first one (not shown). During the autumn-winter period, this condition, as well, made it possible to keep at high levels the overall purification capability of the plant.(*) Values calculated as weighted average according to the incidence of the type of wastewater sampled (domestic, from milking, from washings) on the total inflow value.
The evolution of the concentrations of the various forms of nitrogen through the constructed wetland highlights conditions of diffuse anoxia that do not allow significant nitrification inside the wetlands; this phenomenon happened despite the fact that there were good levels of mineralisation of the organic nitrogen and elimination of total nitrogen.
In the second year, the reduction percentages for COD, BOD, TSS, total N, total P attained the same levels of the first year (data not shown).
The data collected on the development of Phragmites australis showed uniform propagation of the reeds and greater biomass of the epigeal and radical parts, especially in the first bed This is probably due to the larger amount of nutrients reaching the first wetland with respect to the second. n the search for mycorrhizae, only endomycorrhizae were identified, with a clear prevalence of vesicular over arbuscular, without, however, significant differences between the two reed beds (not shown).
Considering the results of the analyses of copper and zinc, with total concentration levels in the plants (culms + rhizomes + roots) in wetlands 1 and 2, respectively, of 10.9 and 6.1 mg/m 2 for copper and 62.7 and 17.8 mg/m 2 for zinc, it was possible to conclude that the two metals were accumulated more by the reeds of the first wetland. Moreover, it can be argued that insignificant percentages (0.5-1%) were assimilated in the plant tissues with respect to the quantity of metals on entry to the wetland from the beginning of the vegetative period.
Microanalysis with SEM/EDX has been carried out on root, rhizome and culm samples. Typically, the most abundant elements in all plant tissues are K, Ca and P. Dot maps of element localization (data not shown) illustrate an interesting feature of Phragmites roots: Fe accumulates at the surface of the periderm, creating a sort of plaque due to oxidation. Tanton and Crowdy (1971) and Peverly et al. (1995) report that this plaque of Fe oxide would determine the accumulation of other metals, such as Cu and Pb,acting as a barrier and preventing their entry into plant tissues. Semiquantitative analyses carried out on cells within plant tissues evidenced that Si is very abundant in the periderm of roots and rhizomes. On the contrary, Ca, P and K are particularly abundant in parenchymatic cells. Cr, Cu, Ni and Pb are more concentrated in ipogeal parts as compared with culms, without specific tissue localization.
A comparison between plants from the two lagoons did not evidence significant differences in the distribution of elements among tissues. Pb was significantly higher in plants from the first lagoon,whereas Cr, K and P were higher in plants from the second one.
Conclusions
Over the last several years, the rediscovery of the environmental qualities of constructed wetland systems have favoured their rapid diffusion, with applications aimed mainly at the treatment of municipal wastewater (Pergetti et al, 1995).
In the livestock production sector, constructed wetland systems can find application in the treatment of wastewater produced during milking operations in the parlour, offering a solution to the problem, still often unresolved, of correct disposal of these wastewater with acceptable costs (such systems can be created for the most part with on-farm work). In perspective, the widespread application of this clean up technique could also respond to the problem of domestic sewage from rural dwellings not connected to the public sewage mains and situated on and/or near farms, particularly on the hills or in mountain zones.
A constructed wetland consisting in two horizontal sub-surface flow beds to treat dairy parlor wastewater and domestic sewage was built in a mountain agricultural settlement in Northern Italy. The beds were planted with Phragmites australis. Wastewater inflows and outflows were sampled throughout two years of the trial. The efficacy of the wetlands was maintained also during the winter months. The wetland operation brought to significant reduction in BOD5, COD, nitrogen, copper and zinc.SEM/EDX analyses evidenced the precipitation of iron on the outside surface of the root,reported to be acting as a barrier towards uptake of other metals.
The creation of the horizontal subsurface flow constructed wetland operating on the wastewater of the Santa Lucia farm not only serves as a demonstration plant but also velops a technique that is well-known though rarely applied to wastewater of this type.
This, along with the monitoring of the operating conditions and the purification performance of the plant, has provided important indications, in particular regarding the most suitable criteria of design, construction, and management of constructed wetland systems serving dairy cattle farms equipped with milking parlours.
The plant monitoring allowed to verify good purification performance, with high percentages of pollutant removal.
The application of SEM/EDX lead to the identification of the heavy metals and macronutrients distribution in plant organs and tissues. These data can help to understand the role of Phragmites in the wastewater clean up process.
What is yet to be verified are the possible applications of this treatment scheme in other important production sectors, first of all that of the agro-food industry related to milk production (wastewater from cheese-making) (Ferrari and Piccinini, 1989), and thepossible system developments that could lead to an improvement in purification efficiency (vertical flow).
REFERENCES
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