POSSIBLE REMOVAL OF SEVERAL CONTAMINANTS FROM THE WASTEWATERS BY A NATURAL BIOFILTRATION PROCESS

Fig. 2 scheme of wastewater pathway in the pilot-plant tank

 

waste inlet

waste outlet

wastewater pathway

Binelli* A., Pogliaghi* A., Zicchinella* M., Marazzi° F., Parolini* M., Soave* C., Mezzanotte° V.

*Department of Life Sciences, University of Milan, via Celoria 26, Milan (Italy) ° Department of Science of Environment and Territory, University of Milan-Bicocca, P.zza della Scienza 1, Milan (Italy)

Introduction and objectives: The aim of this research is the evaluation of possible abatement of some chemicals in sewage waters by the use of a biological process. In detail, we built a pilot-plant (Fig.1) in the biggest depuration plant of Milan (Italy) in which we put tens of thousand specimens of the freshwater bivalve Dreissena polymorpha. Mussels were attached to Plexiglas panels that force the waste to follow a zigzag pathway (Fig. 2). We exploited two natural behaviors of this mussel: the high filtration rate (average of 200 mL/mussel/h), which transfer the contaminants from waste to the bottom of tank by the production of faeces and pseudo-faeces, and the bioaccumulation capability of lipophilic chemicals in mussel soft tissues. Thus, by the cyclic elimination of faeces and contaminated mussels from tank, we may depurate the waste. Our project plan foresee three different steps: 1) the evaluation of the capability of D. polymorpha specimens to attach themselves to panels and to live in wastewater 2) to test the possible decrease of pollutants in re-circulation conditions; 3) to check the contaminant abatement with “open” pilot-plant (without re-circulation). We will measure the possible removal of several environmental pollutants (Tab. 1), which were found in previous studies carried out in the wastewater treatment plant of Milano-Nosedo.

Fig. 1 Pilot-plant located in the depuration plant of Milano-Nosedo  

Re-circulation tank

Pilot-plant

Sedimentation outlet valves

Nursery Attachment tanks

MEASURED PARAMETERS PHARMACEUTICALS: atenolol, carbamazepine, cyprofloxacine, clarithromicyne, dehydro-erythromycin, diclofenac, ibuprofen, paracetamol, furosemide, hydrochlorothiazide, ketoprofen, naproxen, ofloxacin PERSONAL CARE PRODUCTS triclosan DRUGS cocaine, benzoylecgonine, methamphetamine, methadone HEAVY METALS Al, Cr, Fe, Mn, Ni, Pb, Cu, Cd, As PAHs acenaphthene, acenaphthylene, anthracene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(g,h,i)perylene, benzo(a)pyrene, chrysene, dibenz(a,h)anthracene, fluoranthene, fluorene, indeno(1,2,3,c,d)pyrene, phenanthrene, pyrene HYDROCHEMICAL PARAMETERS COD, Total Suspended Solids (TSS) BACTERIA Escherichia coli

Pilot-plant characteristics: In May 2011 we built the pilot-plant (800 L) by stainless steel, which collect the waste directly from the canal came out from the sedimentation tank of the sewage treatment plant. We can also regulate the inlet pump flux (0-5,000 L/h). The bottom of the tank is skew to favor the collection and elimination of sediment and faeces and pseudo-faeces. We can add twenty removable Plexiglas panels in which zebra mussel specimens are attached. An outlet valve allows to put the waste to the sand-filters or to a re-circulation tank (Fig. 1). Moreover, we built also two “attachment tanks” in which Plexiglas panels are put in horizontal to allow the natural attachment of mussels. Finally, the “nursery” is a little tank in which panels can be placed vertically before the positioning in the pilot-plant.

Experimental Design: After several preliminary tests (Tab. 2; Fig. 3, 4), we started to carry out the preliminary clearance tests by measuring the COD decrease due to the filtration rate of mussels in comparison with controls (no mussels in the pilot-plant). The first test was conducted by adding Spirulina spp. to the final waste, since its COD resulted very low (≈10 mg O2/L). After the confirmation of the possible role played by Zebra mussels in the COD abatement (Fig. 5), now we are carrying out several tests to evaluate the clearance of the selected chemicals in the real wastes (Tab. 1). In detail, we are testing four different types of waste: 1) the treated effluent which flows from the sedimentation tank of the waste water treatment plant; 2) the inlet waste of depuration plant; 3) 50% of inlet and 50% of treated effluent; 4) 25% of inlet and 75% of treated effluent. We are measuring the contaminant concentrations every 30 min for 4 hours, in order to obtain the typical removal slope for each measured chemical. These first tests were carried out in re-circulation conditions (Fig. 1) with a waste flux of 3,500 L/h, which corresponds to a residence time of 15 min in the pilot-plant (Fig. 2). The time of complete re-circulation was 18 min since we added always 200 L of selected waste in the re-circulation tank. This experimental design can allow a longer contact of waste with mussels since it can re-circulate about 14 times in 4 hours. Each sample was previously filtered by a bag filter (mesh=1mm) in order to eliminate the gross particulate.

Tab. 1 Chemical and biological parameters measured in the removal tests  

Residence time at 3,500 L/h = 15 min.

>2,000 mussels

ADVANTAGES IN THE USE OF ZEBRA MUSSEL

•  ATTACHMENT OF THOUSAND OF MUSSELS/PANEL

•  HIGH FILTRATION RATE

•  TOLERABLE TO POLLUTANTS

•  HANDY SIZE

POSSIBLE DRAWBACK

•  ALLOCTONOUS SPECIES

•  Zebra mussel is integrated in the Northern Italian ecosystems since the end of 1960s.

•  The pilot-plant is located before the sand filters and disinfection system (peracetic acid).

•  Narrow mesh nets close every outlet.

Preliminary results We show only preliminary results because the first determinations of chemical removal rate comparing controls (without mussels in the tank) with “exposed” are still in progress. Considering the advantages and drawbacks of different attachment systems (Tab. 2), we chosen the “natural” system as the best way to attach mussels. In particular, a high mussel mortality has noticed after few weeks for the artificial systems. By contrast, the natural attachment has guaranteed a very low mortality for several months, even if we noticed a slow time of attachment (two weeks at least) and a weak adherence by the byssus. The second preliminary step has been the evaluation of possible metabolism effects on D. polymorpha due to wastewater exposure. Figure 3 shows a little decreasing trend for lipid percentage and a corresponding non-significant increase of proteins measured in a pool of mussel sampled during a period of about two months. Moreover, a lipid content of about 10% was subsequently measured after three months of placement in the pilot-plant, highlighting the mussel adaptation to physical and chemical wastewater characteristics. In order to check the possible photo- and bio-degradation of pharmaceuticals and drugs of abuse, we firstly carried out four assays with different waste concentrations without mussels in the pilot-plant (Fig. 4), protecting the tanks by photodegration with plastic dark large sheets. No differences were noticed after six hours of re-circulation. The first removal test was the comparison of COD decrease with or without mussels in the pilot-plant, by adding to final waste a content of Spirulina spp. sufficient to reach about 50 mg/L of COD. Figure 5 clearly shows the higher and significant COD removal rate (two-way ANOVA, F=809.9; p<0.001) observed in the experiments conducted with Zebra mussel specimens in the pilot-plant in comparison with blanks (without mussels). The COD decrease (19%) measured in tests without mussels was due only to natural algal sedimentation, while the removal rate due to Zebra mussel filtration increased until a mean of 42% after four hours.

ATTACHMENT TESTS

AQUARIUM SILICONE/GLUE

NATURAL SYSTEM

ADVANTAGES

•  FAST ATTACHMENT

•  HOMOGENEOUS •  STRONG ADHERENCE

•  HIGH VITALITY •  MUSSEL LONGEVITY

•  HIGH DENSITY

DISADVANTAGES •  LOW DENSITY •  MUSSEL MORTALITY

•  SLOW ATTACHMENT •  WEAK

ADHERENCE

Tab. 2 Comparison between artificial and natural systems for mussel attachment to panels.

 

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Fig. 3 Lipid percentage and protein content (±st. dev.) measured in mussel pools maintained in the wastewater.  

Fig. 5 Average COD trends measured by adding Spirulina spp. in the pilot-plant with and without mussels. Difference between tests is significant (Two-way ANOVA, F=809.9, p<0.001)  

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Fig. 4 Concentration of several pharmaceuticals and drugs measured at t=0 and after six hours (t=6).

in=inlet of depuration plant; out=canal from the sedimentation tank. ate=atenolol; carb=carbamazepine; cypr=cyprofloxacine; clar=clarithromicyne; dehydr=dehydroerythromycin; dicl=diclofenac; fur=furosemide; hydr=hydrochlorothiazide; ket=ketoprofen; nap=naproxen; ofl=ofloxacin; coc=cocaine; benz=benzoylecgonine; met=metamphetamine; meth=methadone  

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