Journal of Applied Phycology 7: 479-485, 1995. 479( 1995 Kluwer Academic Publishers. Printed in Belgium.

Utilisation of Ulva rigida biomass in the Venice Lagoon (Italy):biotransformation in compost

V. Cuomo*, A. Perretti, I. Palomba, A. Verde & A. CuomoEcolmare R & D, Via delle Rose 50/a, 80063 Piano di Sorrento, Naples, Italy(*Author for correspondence)

Received 28 July 1994; revised 11 August 1995; accepted 19 August 1995

Key words: Ulva compost, eutrophication; seaweed utilisation, 'green tides'

Abstract

In the Venice Lagoon (Italy), about 106 t (wet weight) of Ulva rigida biomass are produced annually as oneof the major results of eutrophication. Harvests have been initiated to reduce negative impacts of this biomass,however, due to the high costs of such effort, only 40000 t yr- ' are currently being collected. At the moment,biotransformation into compost seems to be the only feasible technology for utilising large quantities of Ulvabiomass. We describe and discuss here a successful composting strategy together with the chemico-physical andmicrobiological characteristics of the resultant composts. Our composting experiments were conducted at a scaleof 20 t. The composting technology utilises large proportions (70-90%) of Ulva biomass and results in a valuable,high-quality end product (compost and compost-based products). This process and the resulting products representsa relatively simple way of utilising the Ulva biomass produced annually in the Venice Lagoon.

Introduction

Seaweeds have been used for many years in agricul-ture, as manure, soil conditioner and as a growingmedium. Stephenson (1974) demonstrated the effec-tiveness of seaweed green manure, mainly due to itshigh concentration of potassium salts, but also to itstrace elements and growth factors such as cytokinins,betaines and auxins (Blunden et al., 1986). In the Ital-ian coastal regions the farmers have for many gener-ations fertilised their fields with macrophytes such asPosidonia oceanica or Zostera spp., and macroalgae,such as Ulva, Cladophora and Fucus. The algae areusually used after fermentation and some months ofrain to eliminate the salt content.

Where large scale harvest of aquatic biomass isundertaken for environmental purposes, disposal orutilisation of the resultant biomass is the major hin-drance to the economic success of the harvesting pro-gram. Ulva rigida has many potential uses; however,most have not proven to be economically worthwhile.For example, although it is a relatively good substratefor bioconversion to biogas (anaerobic bioconversion),

its methane yield and production rate seem quite low(G. Shelef, pers. comm.). This is probably due to thelow biodegradability of Ulva polysaccharides (Mis-soni & Mazzagardi, 1985; Briand & Morand, 1987;Morand et al., 1991).

Biotransformation into compost (aerobic fermen-tation) appears at the current time to be the simplestbiotechnology and probably the most appropriate, froman economic point of view, for supporting a largescale Ulva biomass harvesting biomass. The princi-pal objective of the composting process is the aerobictreatment of biodegradable organic waste in order toobtain humified organic matter for agricultural purpos-es. The starting material may be of any consistency andmay originate from one of many biological sources,either agricultural or urban. The fundamentals aspectsof the composting process have been well defined anddescribed by various authors (Finstein et al., 1980,1982, 1987; Bardos & Lopez Real, 1989; Lopez Real,1991).

Experiments on composting Ulva biomass havebeen carried out in Brittany (France) and in Venice(Italy). In the Brittany experimental trials, Ulva

480

Fig. 1. Schematic pattern of the biotransformation process of Ulva biomass in compost.

biomass was composted with sawdust and bark (Braultet al., 1983, 1985) or with wood or manure (Potoky,1983). In 1986 composting was undertaken at a semi-industrial scale using a mixture of seaweed and woodwaste with the resulting compost-based products beingmarketed to both hobbyistics and professional users(Potoky, 1988; Potoky & Maze, 1988). The main aimfor these researchers was the optimal integration ofalgal biomass with a variety of easily found carbon richsubstrates or animal wastes (Potoky, 1983; Potoky &Maze, 1988; Morand et al., 1990). Preliminary experi-ments were also carried out in Italy by Orlandini (1988)and Cuomo et al. (1991). Quite recently the processhas been simplified in order to reduce the compost-ing treatment costs and to obtain a stabilisation of thebiomass (Maze et al., 1993; Vallini et al., 1993).

The aim of the present study was to optimise aprocess utilising large quantities of Ulva biomass forproduction of a high quality compost suitable for useas agricultural soil improvers or growing media. Theprocess has been defined at 20 t level using as thesubstrate biomass of 70-90% Ulva collected in theVenice Lagoon (Italy).

Materials and methods

Seaweed composting has rarely been studied in detail,so the various processes involved, such as the pattern ofbiological degradation and the role of microorganismsare not known well. Thus many questions arise con-cerning the process for composting of Ulva biomass.

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Table 1. Efficiency of water treatment in removing salts (NaCl) from collected Ulva biomass.

Treatment Time Accumulative Conductivity Calculated Total

(s) water volume of filtrate removed salt removed salt(L kg- ' algae) (mmho cm- ) (g NaCI kg- (g NaCI kg- l

alga) alga)

Immersion 0 0 48. 5 0

(Successive 15 1.3 13.4 4.1

immersion for 15 s 30 2.6 6.7 2.05 7.65with I min of 45 3.9 3.0 0.9

drainage) 60 5.2 2.0 0.6

Continuous spray 0 0 48.5 0

(10.4 L min-1 ) 15 1.3 16.6 5.05

30 2.6 8.0 2.4 9.745

45 3.9 3.4 1.05

60 5.2 2.5 1.25

Continuous fine 0 0 48.5 0

spray (0.775 L 60 0.38 6.7 0.6

min- 1) 120 0.75 8.5 0.75 3.45180 1.14 10.0 0.9

240 1.53 13.2 1.2

Intermittent spray 0 0 48.5 0

(10.4 L min-L) 15 0.5 37.0 4.34

I s on-l s off 30 1.0 28.0 3.3 10.84

45 1.5 16.0 1.9

60 2.0 11.0 1.3

The most important points to be resolved include: theproblem of washing the biomass to remove salts; themanagement or manipulation of optimal C/N ratio; thepossible value of organic additives such as straw, woodshavings, etc. in enhancing the composting process andaffecting the compost's final quality; the possible val-ue of addition of inert materials, such as vermiculite,to enhance the composting rate and final quality; thepotentially serious problem of heavy metal content.

Salt removal was tested under several washing pro-cesses. In the first set of experiments, immersion for15 s in plastic boxes and draining for 1 min on a net. Inthe second experiment, a spray system was tested usinga net where the Ulva was turned and sprayed with acontinuous flow of 10.4 L min- . In the third set, thealgae were sprayed continuously, but with a fine flow(0.775 L min-'). In the last experiment, Ulva waswashed with an intermittent spray flow (1 minute on 1minute off, 10.4 L min-'). The resulting biomass was

analysed for sodium, potassium, calcium, magnesiumand nutrient content.

Since Ulva biomass cannot compost alone (too lowC/N ratio and poor structure), straw (10%) or woodshavings (10-15%) were added to obtain C/N ratiosin the range of 20-40. In parallel, in order to studythe effect of changing the physical properties withoutchanging the C/N ratio, an inert material, vermiculite(5%) was added to one of the treatments. Peat moss(15%) was also added to enhance the compost quali-ty.

Heavy metals were measured using standard IRSA-CNR methods (1985).

Two types of composting technologies were test-ed in bench scale (10 kg) and pilot plant (20 t) units:'induced air windrows' for the fermentation step and'windrow composting' for the maturation step (LopezReal, 1991). In the first, a forced aeration tempera-ture feed-back control system was used. During this

482

stage a thermostatically controlled blower fan is acti-vated to blow continuously until the temperature reach-es the desired level. Fermentation is completed whenthe biomass no longer generates elevated temperatures.During the second stage, aeration is supplied by turn-ing the windrow heaps. The turning regime is selectedin order to ensure final biological conversion of theseaweed substrata. The composting process was con-ducted according to the procedure schematically sum-marised in Fig. 1.

Plant tolerance and phytotoxicity tests were per-formed on the final product according to the Raningerprocedure ( 1992) and the bioassay test of Zucconi et al.(1981).

Results

In our trials, the fermentation stage lasted 3 weeks andthe maturation stage lasted 8 weeks.

Trials of the different rinsing procedures showedthat the intermittent spraying of Ulva biomass, whichis turned over a net, is the most efficient in remov-ing salts from thalli (Table 1). The total salts removedwere calculated to be 10.8 g kg- 1 rinsed Ulva, com-pared to 7.65 g kg-1 rinsed Ulva for the immersionprocedure, 9.75 g kg- t rinsed Ulva for the continuousspray (10.4 L m -1) and 3.45 g kg-' rinsed Ulva forthe discontinuous fine spray.

The physical and chemical (Table 2) and micro-biological (Table 3) characteristics of the compostsobtained were quite similar. The role of different sup-port materials in the substrata can be identified accord-ing to the results (Table 4) and in particular to thedifferences in compost quality. Straw acts as a use-ful support for air circulation and in some ways forcolonizing microbial flora, however, the level of car-bohydrates and their availability is quite low, due to thelong period of time required for the biodegradation ofstraw structure and the bioconversion of polysaccha-rides content. Wood, in this respect, represents a bettersource of carbon. Its structure is more easily degradedand its polysaccharide content is more easily convertedby microorganisms. In fact, the significant differencein fulvic acid in compost deriving from substrata con-taining wood has been noted.

The addition of peat moss increases the qualityof compost both physically and chemically. The aimof vermiculite addition was to find some benefit forUlva composting (G. Shelef, pers. comm.). It absorbsammonium-nitrogen, which subsequently oxidised to

Table 2. Ulva compost: chemical-physical characteristics of com-post obtained after 21 days fermentation and 60 days maturation.

Ulva 85% Ulva 70%

Wood 15% Wood 15%

Peat moss 15%

Moisture (%)

Ash (% dm)

pH

Organic matter (% dm)

Organic carbon (% dm)

Total extractable carbon (% dm)

Humic Acid (% dm)

Fulvic Acid (% dm)

C/N

TNK (% dm)

P (% dm)

K (% dm)

Ca (% dm)

Mg (% dm)

NaCI (% dm))

Conductivity (mmho cm- 1: 10)

Water retention (%)

Density (g cm- 3 )

Cation Exchange Capacity

(meq 100 g-1)

Total solids (% dm)

Cd (mg kg- dm)

Cr (mg kg- dm)

Cu (mg kg- dm)

Ni (mg kg- ' dm)

Pb (mg kg- dm)

Zn (mg kg- dm)

48.5 51.2

54 50.7

7.3 7.45

46 49.3

26.7 28.6

3.9 5.7

1.1 1.41

3.5 4.3

26.7 26

1.0 1.1

0.175 0.175

0.75 0.735.14 5.28

3 2.7

1.07 1.05

5.35 4.85

51 57

0.45 0.42

71

47

1.8

30

55

20

50

160

68.8

46.5

2.1

28

50

24

42

145

Substrata composition in weight

Table 3. Microbial flora of the composting final products (UFC= Unity Forming Colonies).

Microbial flora Ulva 85% Ulva 70%

Wood 15% Wood 15%Peat moss 15%

Total aerobic bacteria 9.9 x 103 10 x 1013

(UFC g- i dm compost)

Actinomycetes 4.7 x 106 3 x 106

(UFC g- dm compost)

Eumycetes 1.2 x 107 5 x 106

(UFC g- dm compost)

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Table 4. Physical and chemical characteristics of composting Ulva using various substrate mixtures.

Substrate Ulva 90% Ulva 85% Ulva 90% Ulva 85% Ulva 70%

Straw 10% Straw 10% Wood 10% Wood 15% Wood 15%

Vermiculite 5% Peat moss 15%

Moisture (%) 45.7 52.5 48.4 48.5 51.2

Ash (% dm) 63.8 70.7 60.4 54.0 50.7

pH 6.8 7.7 7.2 7.3 7.45

Organic matter (% din) 36.2 29.3 39.0 46.0 49.3

Organic carbon (% dm) 21.0 17.0 22.6 26.7 28.6

Total extractable Carbon 1.8 2.3 3.41 3.9 5.70

(% dm)

Humic Acid (% dm) 0.6 1.1 1.10 1.1 1.41

Fulvic Acid (% dm) 0.8 1.5 3.2 3.5 4.3

TNK (% dm) 1.7 1.35 0.9 1.0 1.10

C/N 12.3 12.6 25.1 26.7 26.0

P (% dm) 0.183 0.129 0.107 0.175 0.175

K (% dm) 0.967 0.456 0.771 0.75 0.73

Ca (% dm) 2.93 5.57 4.85 5.14 5.28

Mg (% dm) 2.52 1.62 2.46 3.0 2.7

NaCI (% dm) 1.12 1.1 1.06 1.07 1.05

Table 5. Limits for the heavy metal content in compost products.

Countries Pb Cd Cu Cr Ni Hg Zn

(mg kg- dm) (mg kg-' dm) (mgkg-ldm) (mgkg-ldm) (mg kg-' dm) (mg kg- dm) (mg kg-l dm)

Austria 92 150 1 100 70 60 1 400

Canada 150 3 600 150 60 0.15 500

China 150 5 150 150 25 2 500

Finland 100 3 600 300 100 1 1500

Germany 150 1.5 100 100 50 1 400

Italy 150 3 200 150 50 3 500

Netherlands 120 1 90 70 20 0.7 280

Norwey 100 4 1000 125 80 5 700Sweden 100 2 600 150 100 2.5 1500

Swiss 150 3 150 150 50 3 500

USA NY 91 250 10 1000 1000 200 10 2500

USA EPA 90 500 15 450 - 50 - 900

Raninger (1992)

nitrate, which increases the agricultural value of thefinal products. However, in our experiment, the bene-ficial characteristics of vermiculite were minimised bythe large increase in ash content.

Because of heavy metal content can be a limit-ing factor for the use as growing media, Internationalregulations have been defined for the use of heavy met-

al containing composts as soil improvers or growingmedia (Table 5).

A complete lack of phytotoxicity and the absence ofany inhibiting effect on the germination rate of Lepid-ium sativum seeds indicated when the tested products(composts) had reached maturation and a good finalquality.

484

Discussion and conclusions

More than one million tonnes of wet Ulva are producedannually in the Venice Lagoon (Orlandini, 1988),of which only 40000 t are collected with harvestermachines and disposed of as part of current remediationprograms (Bernestein, 1991). Environmental restora-tion related to the reduction of eutrophication phenom-ena in the Venice Lagoon can be aided by the system-atic collection of biomass collected on the growth ratebasis (Frederiksen, 1987; Morand et al., 1992; Cuomoet al., 1993). The composting procedure that we havedescribed, in which large proportions of Ulva (70-90%) are mixed together with a sufficient proportionof wood and peat moss to give a good compost quality,would seem to be the most useful and inexpensive wayof utilising and disposing of Ulva biomass produced inthe Venice Lagoon.

In Brittany an advanced full scale realisation ofalgal biomass composting is being carried out, butwhich uses a smaller quantity of Ulva biomass andrequires more time (7-9 months) for the compostingperiod (Morand et al., 1990). A stabilising compostingprocess of Ulva with a minimum carbonated substratahas been achieved recently by Maze et al. (1993). Insome trials carried out in Italy, composting processis based on a rapid fermentation (21-28 days) withpoor aeration and continuing turn offs with the aim toachieve a stabilization of the biomass together with areduction of volume (Vallini et al., 1993).

Utilisation of compost in agricultural soil is anexcellent disposal method and also provides an organ-ic substrata for microbial flora. However, the heavymetals content could be a limiting factor. However,now the compost from Ulva biomass is also admittedby CEE (Reglement CEE n 2092/91 du Conseil du24 juin 1991).

Our compost deriving from Ulva biomass collectedin Venice Lagoon has chemical, physical and biolog-ical characteristics comparable with the commercialsoil improvers and growing media presently on themarket (such as peat moss and bark products). Its usein domestic, professional agricultural and specialistfields such as horticulture, floriculture etc., crop pro-duction, is a useful way to integrate organic matter incultivated soils, limiting the use of mineral fertilisersN and P and reducing the nutrients lost in water, thuscontributing to efforts at reducing the eutrophicationthat causes the excessive macroalgal growth.

Biotransformation in compost at present representsthe fastest way to use or dispose of Ulva biomass and

could provide the economic support for the systematiccollection of Ulva biomass from Venice Lagoon andsimilarly affected coastlines in other regions.

Acknowledgments

The authors acknowledge Dr John Merrill for criticalreview of the work and Mrs Elisabeth Smith for kindlytyping the manuscript.

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