Industrial Crops and Products 6 (1997) 201 – 212
Cascading of renewable resources hemp and reed Peter J. Fraanje * IVAM En6ironmental Research Uni6ersity of Amsterdam, P.O. Box 18 180, 1001 ZB Amsterdam, Netherlands Received 19 April 1996; accepted 12 June 1996
Abstract Resource-cascading, the sequential exploitation of the full potential of a resource during its use, is one of the ways to improve efficiency of raw materials use. To design a good cascade knowledge of the typical properties and structures of the plant is useful. In this article it is shown that cascading of the renewable resources hemp and reed can lead to large savings in primary resource use. Resource cascading can therefore make the option to replace non-renewable resources by renewables more realistic. In comparison with using biomass (e.g. reed, hemp, straw) for energy directly, cascading also means that, carbondioxide emission, is postponed. This is interesting in relation to global warming. © 1997 Elsevier Science B.V. Keywords: Reed; Hemp; Cascading; Renewable resources
1. Introduction Current industrial production is to a large extent based on (virtually) non-renewable resources. Since the onset of the industrial revolution the use of virgin natural resources has increased strongly, especially after the Second World War (Meadows et al., 1972, 1991). A large share of these virgin resources originated from slow geological processes and can therefore be seen as virtually nonrenewable. The current intensity of use of virgin natural resources leads to resource depletion, loss * Tel.: + 31 20 5255080; fax: +31 20 5255850; e-mail:
[email protected]
of biodiversity and pollution (Reijnders, 1995). This is not in line with sustainable development as defined by the UN-Commission on Sustainable Development (WCED, 1987). For sustainability in the long term it is first of all necessary to decrease the material intensity of our activities (by a sufficiency policy) and furthermore to replace a part of the non-renewable resources by renewables. These renewables include a variety of resources like flax, hemp, reed, crambe, seashells and wood. Based on the concept of sustainability in the Netherlands there is a case for a reduced use of virgin natural resources and a relative shift from virtually non-renewables to renewable resources (VROM, 1989, 1990). An
0926-6690/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 6 6 9 0 ( 9 7 ) 0 0 0 0 9 - 5
202
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
Fig. 1. A resource cascade and conventional resource use in industrial countries (Fraanje and Lafleur, 1994): Q, resource quality; T, utilization time; DT, life time per application; DT, overall life time; DQ, quality loss per application.
expanded use of renewables, following from the replacement of non renewable resources ceteris paribus can lead to an unacceptable pressure on land and water resources. Land for non food applications should not displace land resources necessary for food. Moreover renewable resources can be subject to over-exploitation. Sustainable use of renewable resources on the input side means at least that harvest rates should equal regeneration rates (Daly, 1990). To reduce, or rather avoid pressure on land resources, the efficiency of resource utilization should be increased. Resource-cascading, is here defined as the sequential exploitation of the full potential of a resource during its use and is one of the ways to improve efficiency of the raw materials use. The word cascading originates from the analogy of the cascade of a (mountain) river, where the water is descending from one level to the next, towards the sea or a lake (Sirkin and ten Houten, 1993). The concept of resource-cascading is a bottom-up approach which can help designers and policy makers find ways to a more efficient use of raw materials. This article deals with cascading of the renewable resources hemp and reed and exemplifies
some of the potentials of cascading of renewable resources.
2. Cascading of renewable resources In Sirkin (1991) and Sirkin and ten Houten (1993, 1994) the theory of cascading is described extensively. Based on experience with the practical application of cascading (Sienknecht and Fraanje, 1992; Fraanje and Lafleur, 1994) a simplified approach is presented here. Whereas Sirkin and ten Houten employ four dimensions (quality, time, consumption rate and salvability). Here, I use two dimensions, quality and time. A similar simplification has also been suggested by Sirkin and ten Houten (1994). This simplified approach appears to be suitable to design a resource conservation policy strategy. The principles of (renewable) resource cascading are (see Fig. 1 also): (i) appropriate application (high Q). Appropriate application means that the resource is applied on the basis of its (typical) properties, at the highest quality level that is possible. In this view one should not make pulp out of a certain rigid vegetable structure directly,
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
but first profit by specific and intrinsic qualities of the raw material (like strength). Many industrial applications of vegetable resources exploit the intrinsic value and properties of the original plant to only a limited extent. Burning of biomass for energy should preferably be the last step in a cascade. Appropriate application implies whole resource utilization. Thus in case of hemp or reed not only the stalk should be taken in consideration, but also the use of seeds, leaves, roots etc. (ii) Life time extension (increase DT and DT). The DT (life time per application) can be extended through: 1. optimal design of the product, e.g. by finding an optimal mix of long and short hemp fibres, a long lasting cloth can be produced; 2. optimal application of the product: this aspect is less strongly related to the product, but can be influenced by giving prescriptions or conditions for application. A roof covered with reed should be built under a rather sharp angle, designed in such a way that water cannot easily stay on it, thereby preventing rot; 3. good maintenance of the applied product, aimed at postponing the replacement of the product by using small quantities of energy and raw materials (repair, cleaning). Increasing DT (accumulated life time of all applications) may be helped by increasing the number of steps in the cascade. If maintenance cannot meet the technical requirements anymore, if too much energy and materials are required, or if it brings about too much pollution, then one may look for an application lower in the cascade. (iii) Quality-conservation (minimize DQ). For a next step in the cascade, a next application should minimize quality-loss. An example of this principle is that cutting valuable long stalks into standard short pieces should be avoided. It would be better to first use the long stalks and as a next step the pieces. Cascading is about starting at a high Q (Q= resource − quality), increasing DT ( = life time per application) and DT (overall life time) and minimizing DQ (quality loss per application) (see Fig. 1). The actual use of resources is often in contrast with the ideas of cascading: for instance straw or reed with good (construction) qualities is inciner-
203
ated or pulped (in Fig. 1 this can be shown as a lower Q than the potential). Therefore in case of conventional use, the area under the curve is usually relatively small (see Fig. 1), this in opposition to the area under the curve when cascading. The nodes shown in Fig. 1 represent the moments where choices should be made. The utilization time of the resource in a certain application can be extended at cost of investing (non-renewable) energy and other resources. Another possibility is to find the next application and in doing so, minimize the loss of quality. To optimize the environmental choice, at every node the decision maker should study which is the best solution. In doing so one should think ahead, because some choices block other applications in the future. At the time reed is cut into pieces its properties change and certain uses or applications are not possible anymore or need at least investment of (non-renewable) energy and materials. From the whole stalk to chopped stalkpieces of 10 cm to chips and finally to fibres, the number of possible applications declines irreversibly. Reed fibres subsequently cannot be used anymore as a thatching material or as a carrier for plaster. In practice, the differences between the options will often be clear and if not, determination of the parameters energy and (other) resource-uses can be helpful.
3. Structure For renewable resources a definition of resource quality could be the extent to which the original functional properties are present. The quality of a resource depends on: the energy embodied in the resource; its chemical composition (e.g. oil, cellulose, Sicontent etc); its organization or structure. In the case of a renewable resource like hemp or reed, solar energy, nutrients and water contribute to develop a specific structure with certain properties like strength, durability, oil content, etc. some of which are useful to man. In general the dispersion of matter and/or energy and loss of organization, or more specifically, its structure, can be seen as a loss of quality of the resource.
204
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
According to the second law of thermodynamics, the entropy increases. The first step in designing a good cascade for reed and hemp, is to study properties and structure of the whole plant. A study of the physiological, morphological and anatomical features of the plant and details about the biotopes can be useful to find environmentally sound applications. In this article, attention is focused on the typical structure which reed and hemp, both vascular land plants, have in common: a strong and flexible stalk of about 24 m. It is, in concordance with harvesting methods in the Netherlands, assumed that leaves, roots (and seeds) remain on the land, contributing to the improvement of soil structure. A study of historical uses of plants also proved to be helpful for designing cascades (Fraanje, 1995). In past times, man was forced to use resources efficiently and did not have the means to intervene deeply in the original, natural structures of plants. Traditional applications of vegetable plants can be of relevance now, when searching for products and processes with a low environmental impact. (Fig. 2, Backe, 1936). In the next sections, considerations pertinent to structure and historical use of reed and hempstalks will guide the design of possible cascades for reed and hempstalks.
4. Reed Reed (Phragmites australis) is a residual monocotyledon which is common all over the world. Every year reed forms 2 – 4 m tall shoots, originating from underground stems called rhizomes. Reed flowers in Holland in August in the form of a plume on top of the stalk. For users, an important physiological process takes place at the end of the season: the long green leaves fall off, as the above parts of the plant die after frost while the reedstalks become woody. Fig. 3 gives a schematic drawing of reed. Morphological examination of reed reveals that reed stalk, as in many other monocotyledons, contains pronounced nodes and internodes, which gives reed a high strength, with a minimum of fibre mass (Reinders, 1964). Tests on strength and
flexibility have shown that nature is superior in the optimal combination of these properties (Reinders, 1964). Anatomically, reed stalk is typically hollow except for the nodes, where thin partitions are formed. In Fig. 3 a cross-section of the reed stalk is given (Tobler, 1943). The reed fibre which can be derived from the stalk, is short and not very strong (Yu, 1983). Reed grows in a great variety of biotopes. There are different ecotypes of reed with very different properties (Tobler, 1943; Raghi-Atri and Bornkamm, 1979), resulting in reed with different use-qualities. In the Netherlands reed is harvested mostly from natural stands, but can also be cultivated and is successfully used to prepare the new soils of Dutch polders for agriculture. Sienknecht and Fraanje (1992) review the great variety of traditional and innovative use of the whole or parts of reed plants. As mentioned earlier we focus here on applications of the reedstalk. An important application of reed stalks is reed for thatching, not only in the Netherlands, but also in Germany, Belgium, the UK, France and Denmark. Typical quality demands by thatchers are that the stalks should be straight, 1 year old, thin, hard and white (Sienknecht and Fraanje, 1992). The fact that reed stalks are hollow makes them suitable for roofs, because the stalks are ventilated and therefore dry quickly which makes them less vulnerable to rot. Reed with a high silicium content adds to the durability (Sienknecht and Fraanje, 1992; Wieringa, 1990). Recent research showed that reed harvested from rich soils has less nodes per length unit than reed from poorer grounds. This fact influences the durability of a thatched roof immediately: reed stalks break, when rotting, at the nodes. The longer the internodes, the sooner a thatched roof has to be repaired (Berswordt-Wallrabe, 1995). The Dutch production of high quality reed for thatching in 1992 was estimated at 5500 t, while yearly 10 000 t of reed is applied on roofs, which means that Holland has to import reed for thatching. Every year an area of 600 000 m2 is covered with reed, of which one quarter covers new houses (Sienknecht and Fraanje, 1992). This last figure shows that reed is still a popular roofing material, especially because of its esthetic value and—when
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
Fig. 2. Example of the traditional uses of the whole hemp plant.
205
206
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
Fig. 3. An illustration of (parts of) the reedplant (Diels, 1918) and a cross-section of the stem (Tobler, 1943): L, lumen; V, vascular bundles surrounded with bast fibres.
applied in a modern way — also because of its comfort and relative environmental friendliness. Reed as a thatching material can be considered as a high quality application. In thatching, an optimal profit is made of the strength and size of reed stalks; by making bundles of reed this strength is even multiplied. For about 4 years one thatching company in the Netherlands has been applying reed in a more modern way, in a so called ‘closed and bounded way’, which means that bundles of reed are fastened on board with steel wire and a screw. In the traditional way, reed is fastened with nails on a structure of wooden sticks. In comparison with the traditional way, modern reed roofs are wind and (more importantly) fireproof, which makes it
competitive with other roofs. The new thatching system is used in renovation projects and the building of new houses. In a project in IJsselstein, near Utrecht, new thatched houses were allowed to be built quite close together. In costs, the whole roof was not much more expensive than other roof systems. Traditionally, reed is replaced totally after at least 30 years or more and dumped, but part of this reed can be used again, after cleaning and cutting away the rotted ends. In this way about 70% of the old reed can be reused (Sienknecht and Fraanje, 1992). In Fig. 4 a possible cascade for reed is shown. The cascade for reed stalks is mainly governed by the phenomenon that the stalks become shorter with every step. The graph shows clearly how the
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
207
Fig. 4. Example of a cascade of reed stalks of thatching quality (Sienknecht and Fraanje, 1992).
surface under the curve can be augmented, which means that the resource reed is used in a more effective way. By following this cascade, the traditional life time of reed for thatching is expanded from 30 to over 80 years. All steps in the cascade are a proven possibility, except for the production and use of reed fibreboard which as far as the author knows, is not produced now, but is in theory possible. One author mentions monocotyledons in general as ideal resources for fibreboards (Hesch, 1993). Boards out of bagasse and maize stalks for instance have been successfully produced. In theory a second use as reed fibreboard could well be possible (not shown in the graph). After use, reed can finally be burned with energy-recovery, or composted. For reed with low thatching quality, another cascade can be designed. The whole stalk can be used as a carrier for plaster. For this application, strong and flexible reed is demanded. Recently there is, as a result of a growing interest in sustainable building, a new demand for reed as a carrier for clay plaster. This product is imported now in the Netherlands. For the production of
pressed reed boards long clean reed stalks are required. Pressed reedboards are applied in fences and are produced in the Netherlands and still popular (Sienknecht and Fraanje, 1992). In the case of application of reed as a fibre-resource for paper production, a low silicium content, ‘Papierrohr’ (Tobler, 1943) is necessary (Yu, 1983). Another possible use of reed stalks originates in application of pieces in an insulation element (see Fig. 5). For such an element all kinds of reed could be used, also reed which has been used already and which is too short to use a second time as roofing material. At present, only a prototype of such an insulation element exists, but this innovative application is a good example of a maximum profit of the insulation value of the reed stalk. The chopped reed stalk pieces insulate better in a direction square with the insulation element than parallel, as the pith of the reed stalk has the best insulation value. This application could also be a step in the cascade shown in Fig. 4 (after application on the roof). After use the insulation could be further cascaded and used in reed fibreboard and after that as a fuel with energy recovery.
208
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
5. Hemp Hemp (Cannabis sati6a L. 6ar. 6ulgaris) is an annual, from origin dioecious, dicotyledon (Hayward, 1938) which grows rapidly to a height of 2.5 – 3.5 m in about 100 days (Kirby, 1963). The young stem is succulent, but lignifies rapidly, resulting at maturity in a rigid, woody stem. The leaves are palmately compound, with 5 – 11 leaflets, 7–9 being the usual number (Hayward, 1938). In the Netherlands a monoecious variety of fibre hemp is cultivated (Fraanje, 1995). Unlike reed, there are no specific ecotypes of hemp, but due to diversification through cultivation which started at least 5000 years ago, there are many varieties of hemp. This article deals with fibre-hemp which is densely sown for the production of fibres and develops one shoot without branches which at the time of the harvest consists of a woodpipe containing short fibres and a husk with long fibrebundles. As hemp is a dicotyledon, the anatomy of hemp
Fig. 5. A possible application of reed with or without thatching quality: reed-insulation in prefab wooden wall-elements (Sienknecht and Fraanje, 1992): 1, reed; 2, masonite; 3, gypsum board; 4, wood; 5, fibreboard; 6, ventilation.
is quite different from the anatomy of the monocotyledonous reed and results therefore in uses completely different from that of reed. The long and strong fibre bundles of the hemp plant were so valuable that man already in ancient times found ways to extract these fibres from the rest of the plant. Fibre hemp stems consist of a high-cellulose low lignin bark containing long fibres and a low cellulose high-lignin core containing short fibres; both parts of the plant are so different that they can be considered as two types of raw material (van der Werf et al., 1994). The bark tissue surrounding the central woody column can be separated by mechanical force and/or a natural, chemical process called resting (Goulding, 1919). Hemp was a very important plant and raw material for the Netherlands for centuries, particularly during the Golden Age. Hemp was used for sails, fishing nets, as rope and as oakum. Production of paper out of old linen, hemp and cotton clothes used to be the normal and dominant way of making paper until about 1850 (Wiesner, 1921). Since then, hemp cultivation has declined; worldwide about 200 000 ha of fibre-hemp is grown today. In the Netherlands one windmill, ‘the Schoolmaster’ near Amsterdam makes paper in the traditional way with the traditional resources on a small scale, mainly for artists. Since 1994 fibre hemp is being grown on Dutch fields again. The area under hempcultivation is small, almost 1000 ha in 1995, but forecasts suggest that the area under fibre-hemp in Holland and in Germany will grow strongly in the near future, not in the least because there is a public demand for locally grown fibreplants and environmentally friendly clothes and so-called tree-free paper (Fraanje, 1995). For a more extensive review of the different possible uses of hemp, see Fig. 2 or Fraanje (1995). In the following Figs. possible cascades for the two parts of the hemp stalks, namely husk fibres and woodpipe, are shown. The cascade for husk fibres is mainly governed by the phenomenon that the long fibres of hemp become shorter every step. The long and strong husk fibres can be used in high quality papers (e.g. paper money) and as an upgrader in secondary pulp for paper (Judt,
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
209
Fig. 6. Illustration of parts of the hemp plant (Diels, 1918) and cross-section of a mature fibre-hemp stalk (after Hoffmann, 1957).
1992). This last option is chosen in the cascade of Fig. 7. As far as strength-properties are concerned, 50% of the strength of paper fibers is lost after five cycles (Read, 1993), so the fact that hemp fibres are used three times is quite realistic. The amount of recycled post consumer writing and printing paper is still very low in the Netherlands, at about 2% of the total Dutch paper consumption (Fraanje and Lafleur, 1994), so there is an enormous potential for reuse of old paper on a high quality level, by upgrading old paper through adding (virgin) strong hemp fibres. In Lafleur and Fraanje (1995) a plea is made to use fibres from hemp and flax as a second resource for paper production instead of only woodfibres. Potentially, the production process to extract the huskfibres of hemp causes less ecological damage than the production of wood fibres (van der Werf et al., 1994; Lafleur and Fraanje, 1995).
By using hemp and flax as a resource for primary paper production, the pressure on forests can be reduced (Fraanje and Lafleur, 1994). In Germany, pulp for paper is made from hemp imported from Spain and mixed (50–50) with post consumer paper for sale in Germany and the Netherlands. Reuse of secondary fibres in writing paper is possible, as earlier mentioned. After use in newspapers, hemp fibres and other cellulose can be processed to paperwool insulation. Stimulated by government policy aimed at sustainable building, a growing number of Dutch houses are insulated with this paperwool. In theory paperwool can be used again, after shaking up the material and mixing it with some new material (not shown in the graph). In the end the paperwool can be composted or incinerated in combination with other feedstock. When exploiting the full potential of cascading the total period that hemp fibres are used can be
210
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
Fig. 7. A possible cascade for husk-fibres of hemp.
expanded from about 2 years to over 60 years. All single steps in the cascade shown above are separately proven. The woody pipe (H in Fig. 6) of the stalk, also contains fibres, but of shorter length and are more difficult to extract. From an environmental point of view it is preferable to find a good application for the woody chips as they arise from the extraction of the fibres. Woody parts of the hempstalk, the hemp strives, can be used in combination with a simple resin to make hemp based chipboard (see Fig. 8). Hempshives are traditionally incinerated or used in combination with flax strives and woodchips for the production of chipboard. In the Netherlands there is one company which produces 100% flax chipboard. With the same process a chipboard of hempshives could be made. By applying such hemp based chipboard in a standard roof element or partition wall with standard sizes, the product can be used a few times over again. Usually, a house or office is renovated about every 25 years, though technically it can last much longer. In theory, the chipboard could be processed and used to make fibreboard, but here it is assumed that after being used three times, the hempbased chipboard is incinerated with energy recovery.
When applying a cascade for hempshives as shown in Fig. 8, the total use time increases to at least 75 years, instead of incineration immediately after harvest. One step in the cascade shown in Fig. 8 is technically proven, i.e. the making of 100% hemp chipboard. Developments in the Dutch building sector over the past years have shown that more and more, prefabricated roof elements and replaceable inner wall systems are being employed. In theory, a hempbased chipboard could be used in such elements.
6. Conclusions and recommendations Cascading can strongly increase the overall life time of the renewable resources hemp and reed. For reed stalks, a cascade of three to four steps can be set up, which extends the lifetime of reed stalks from 30 to 80 years. For husk fibres of hemp the lifetime can be expanded from about 3 to 60 years or more and cascading of hempshives can result in a use of 75 years instead of immediate incineration. By cascading resources, the efficiency of resource use increases significantly. Cascading can
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
211
Fig. 8. A possible cascade for hempshives from the woodpipe.
be helpful as a planning tool for a more sustainable and efficient use of renewable resources like hemp and reed. Most single steps of the cascades presented are a proven possibility. However, only parts of these cascades are now being put into practice. If a great deal of reed for thatching would be cascaded, this could mean that the import of reed in the Netherlands could be decreased. In the Netherlands and in some other European countries, there is a large deficit of (wood) pulp for paper on one hand, while on the other hand there tends to be a surplus production of food crops. Therefore, a recommendation for the Dutch and other European governments that may limit both food surpluses and imports of woodpulp, is to expand the cultivation of fibre hemp and use of hempfibres to upgrade old paper and to promote post consumer recycled paper. By applying resource cascades on a large scale the shift from non- renewable resource usage towards a greater share of renewables can become more realistic. Cascading of renewable resources like hemp and reed, but also straw, flax, miscanthus, etc. on a large scale can also be interesting as a means of limiting atmospheric concentrations of
the ‘greenhouse gas’ carbon dioxide. As long as the products are in use, the carbon is locked up and C02 emissions are postponed; this is relevant to limiting global warming. More attention to the original properties and structures of plants is desirable and preferred above the now dominant approach of using nonfood plants as a pulping or energy resource. Currently most of the attention for vegetable resources is aimed at using the biomass for direct incineration. In a cascading-strategy for non-food vegetable resources, incineration or composting is usually the last step in a cascade after (many) other applications. Studying the historical, ecological, morphological, anatomical and physiological aspects of plants is useful to detect high-quality applications of renewable resources. It is striking that for information about the use of plants as a renewable resource for non-food purposes, mainly quite old literature is available (e.g. a standard work from Wiesner, 1921). One of the reasons for this could be the fascination of many researchers for synthetic products and/or new materials. It would be very useful to intensify the research on vegetable resources, as these are widely seen as major resources for the future.
P.J. Fraanje / Industrial Crops and Products 6 (1997) 201–212
212
Acknowledgements The author thanks Professor Dr L. Reijnders of the Interfaculty Department of Environmental Sciences (IDES) of the University of Amsterdam and the anonymous reviewers of this journal for their comments, Hempflax, producer of fibrehemp and flaxproducts in Oude Pekela, W. van Dijk BV a thatching company in Putten and Tjerk Reijenga, architect of BEAR in Gouda.
References Backe, H., 1936. Der Hanfanbau. Parey Verlag, Berlin. Berswordt-Wallrabe, Th. von., 1995. Riet voor dakbedekking. LUW vakgroep Agronomie, Wageningen. Daly, H., 1990. Towards some operational principles of sustainable development. Ecol. Econ. 2 (1), 1–6. Diels, L., 1918. Ersatzstoffe aus dem Pflanzenreich. Stuttgart. Fraanje, P.J., Lafleur, M.C.C., 1994. Verantwoord gebruik van hout in Nederland. (Sustainable use of wood in the Netherlands) IVAM Environmental Research, nr. 94/08. University of Amsterdam. Fraanje, P.J., 1995. Vezelhennep voor de bouw (Fibre-hemp as a building material). IVAM Environmental Research, nr. 95/06. University of Amsterdam. Goulding, E., 1919. Cotton and other vegetable fibres-their production and utilisation. John Murray, London. Hayward, H.E., 1938. The Structure of Economic Plants. Verlag J. Cramer (reprint 1967), New York. Hesch, R., 1993. Reproduzierbare Rohstoffe fur die Holzwerkstoffherstellung. Holz–Zentralblatt, Stuttgart. no.7 & 10. Hoffmann, W., 1957. Flachs und Hanfanbau Deutsche Bauernverlag Berlin, DDR. Judt, M., 1992. Set to make a come back in furnishes. PPI, August 1992. Kirby, R.H., 1963. Vegetable Fibres. Interscience, New York. Lafleur, M.C.C., Fraanje, P.J., 1995. Papier uit hennep en oud papier (Paper from hemp and old paper). IVAM Environmental Research no.95/03. University of Amsterdam.
.
Meadows, D.H. et al., 1972. Grenzen aan de groei (Limits to growth). Het Spektrum, Utrecht. Meadows, D.H., Meadows, D.L. Randers, J., 1991. Beyond the limits. Chelsea Green, Post Mills, Vermont. Raghi-Atri, F., Bornkamm, R., 1979. Wachstum und chemische Zusammensetzung von Schilf (Phragmites australis) in Abhangigkeit von der Gewassereutrophierung. Arch. Hydrobiol. 85 (2), 192 – 228. Read, B., 1993. Secondary fiber: a primary resource? PPI October 1993, 4748. Reinders, E., 1964. Leerboek der plantkunde. Scheltema and Holkema, Amsterdam. Reijnders, L., 1995. Environmentally Improved Products and Production. Kluwer, Dordrecht. Sienknecht, K., Fraanje, P.J., 1992. Bouwen met riet (Building with reed). W/E, Gouda. Sirkin, T., 1991. The theory of cascading; a design and planning tool for appropriate resource usage. Center for Human Ecology, Aarhus University, Aarhus. Sirkin, T., ten Houten, M., 1993. Resource cascading and the cascade chain. IVAM no. 71, University of Amsterdam, Amsterdam. Sirkin, T., ten Houten, M., 1994. The cascade chain. Res. Con. Recycling 11, 215 – 277. Tobler, F., 1943. Stengelbau, festigkeits – und Verwertungsunterschiede beim Schilfrohr. Angewandte Botanik 25, pp. 165 – 177. VROM, 1989. Nationaal Milieubeleidsplan. TK 1988 – 1989 21137, nrs. 1 – 2, Den Haag. VROM, 1990. Nationaal Milieubeleidsplan Plus; bijlage 2 Duurzaam Bouwen, TK 1989 – 1990 21137, nrs. 23, Den Haag. van der Werf, H.M.G., et al., 1994. Quality of hemp (Cannabis sati6a L.) stems as a raw material for paper. Ind. Crops Prod. 2, 219 – 227. Wieringa, P.C., 1990. Het weke dak. Rijksdienst voor Monumentenzorg. Zeist. Wiesner, J. von., 1921. Die Rohstoffe des Pflanzenreiches -Versuch einer technischen Rohstoffflehre des Pflanzenreiches. 3te Auflage, dritte band, Leipzig. World Commission on Environment and Development, 1987. Our Common Future. Oxford University Press, Geneva. Yu, Y., 1983. Reed pulping in China. Nonwood Plant Fiber Pulping. TAPPI Progress Report no. 14, pp. 21 – 23.
.