Pages 129-137 without figures


Principle 

The so-called sap-replacement treatment provides the best protection for bamboo culms regarding both culm penetration as well as the safe use of the treated culms. It is generally known as the “Boucherie-process” after the French inventor Auguste Boucherie (1840) and has been applied long since for the treatment of freshly felled trees used for telephone poles. In its conventional form the preservative solution (e.g. CuSO_4, ZnCl₂) is forced by pressure into the butt-end of the trunk with the bark intact whilst the sap drains away from the top end (due to a hydraulic pressure of 1 bar originating from a vessel with the preservative placed at 10 m elevation, e.g., a tower) which is connected by pipes to the butt-end of the stem. This gravity system is still practised by placing a tank high enough to create sufficient hydrostatic pressure. 

Purushotham et al. (1954) developed the system for bamboo known as the “Modified Boucherie Treatment” using first a simple bicycle pump (Fig. 95). The method was investigated further during a FAO consultancy 1957/58 (Liese 1959) regarding its influencing parameters, like culm age, moisture content, direction of flow, preservative, concentration, and pressure. The improved method is now widely used in several countries (e.g. EBF 1994; Gonzales and Gutiérrez 1996; Liese et al. 1998; Cusack 1999). For a better understanding of the principle, the term “sap-replacement treatment” appears more appropriate. The principle of the process is based on a pressure pump that pushes a preservative solution through the entire length of the culm, so that the sap in the vessels is replaced by the preservative. Sap-replacement is a safe and environmentally friendly treatment for bamboo culms as the preservative remains entirely inside the culm. The clean surface does not cause any harm when touched.



Several parameters have to be considered for a successful treatment


Requirements and Preparation of Bamboo Culm

The bamboo to be used should be about 3-4 years old. Younger culms with a higher moisture content are easier to treat but tend to collapse and split during subsequent drying; older ones may have insufficient moisture. The culms must be free of defects, like lesions and insect holes, which will block the pathways. The bamboo must be treated promptly, on the day of harvesting and consequently cutting, transport and treatment have to be well organized. 

Before treatment, the culms should be kept under shade and cross-ends should be covered with damp cloths to stop them drying out. Ideally the culms should be cut to size only at the treatment site to avoid additional open ends which lose moisture and suck in air. If a cutting has to be done at the forest for transport reasons, one extra internode should be left on both ends to reduce water loss and cut-off later. Just before the culm is clamped into the nozzle of the pipe a fresh cut of some centimetres has to be made at both ends with a very sharp knife to guarantee water-filled vessels with no air pockets. A fine saw might cause tattered fibres to block the vessels. The cut should be made near a node using its irregularity to ensure a better fastening of the rubber sleeve. If a culm portion is to be treated where branching begins, a stub of about 30 cm should be left so that no preservative is likely to leak out. After treatment the stubs should be cut off and disposed of accordingly.

If the bamboo cannot be treated within 12 hours of harvesting a soaking in fresh, clean water in a tank, pond or stream is needed. A storage time of some days will additionally moisten the bamboo tissue and is recommended in the dry season when culm moisture is naturally low. Given that the culms float easily they have to be kept submerged with sinker loads (Fig. 96).

Before treatment the amount of culm volume should be calculated to obtain an esteem of the time required. The figure is obtained by measuring the average outer culm circumference (at the base and at the top) multiplied by culm length and subtracting the average inner circumference multiplied by culm length. As the vessel volume is less then 10% of the culm wall a treated volume of 10%, as measured by the outflow in cubic cm, would ensure this is accomplished. Vessel volume varies between species and their origin and therefore determines the speed and time of the flow. A study on two Guaduas from Costa Rica has shown distinct differences in vessel area between the two which could account for the different treatment times (Liese et al. 1998).

Preservative

The water content of the vessels is replaced under pressure by a preservative with the result that the culm is “treated”. However, to achieve such protection the vessels’ surroundings must be penetrated by a subsequent diffusion. Therefore, non-fixing salts are more suitable than fixing salts, whereby boron-salts are most commonly and successfully used. Their concentration ranges between 5-10%. In Costa Rica a boron-based fertilizer, Menorel, has been economically applied. CCB- type (6-10%), CCA- type (5-8%) and copper sulphate (10%) are also used successfully but require longer treatment times than boron salts.  Fixing-type preservatives may cause a blockage if the culms are not sufficiently fresh. With increasing concentration the time allowed for sufficient penetration has to be extended.

From the outset the preservative has to be carefully mixed with clean water and stirred, and the required concentration checked by pH meter or hydrometer. As solution impurities will hinder flow it has to be filtered before filling the container and again just before entering the manifold system of the culms. A double layer of nylon filter cloth or a fine online filter with a paper filtering element is effective. If the treating solution is not coloured (like copper sulphate) a suitable dye, like aniline textile dye could be added, so that the modified solution can be recognized at the top end of the culm. The colouring may reduce the reaction of boron to turmeric/curcumic acid, which turns red with sufficient quantity.

Treatment Equipment

The treatment equipment is shown in Fig 97, as modified from a sketch by the EBF (1999). It consists of:

A   closed container for the treatment solution to take pressure of 1.5 bar 

B   tap for regulating the input of solution, 

C   water pump machine or air compressor 

D   tap for regulating the output of solution 

E   pressure regulation tank

F1-F6 tap for regulating the solution flow to the nozzles of the manifold outlet

G   collection tank for surplus preservative

H   tap for bleeding air from the nozzles before solution enters 

I   nozzle with rubber sleeve and clamps to seal the pipe-culm joints

J   collection of outflow

K   bucket for solution

The tank must be able to stand the pressure applied. As the preservative may be corrosive, heavy-duty plastic should be used for all components.

 

Treatment Procedure

As in any other treatment operation the workers have to observe the safety instructions 

The butt-end (never the top) of the freshly trimmed culm is clamped into a rubber sleeve tightly above a node to avoid it slipping. There are several types of rubber sleeves and plastic fittings with clamps in use (Fig. 98). It is most important to secure a tight seal. A long tapered rubber nozzle can adapt to a wide range of bamboo diameter, but should be covered by a sleeve of fibreglass or wrapped in cloth to avoid ballooning of the rubber sleeve. When the pump is turned on the pressure will first withdraw the air in the pipe through a bleeding valve. Only when the solution flows into the bucket can the bleeding valve be closed so that the liquid is pushed into the water-filled vessels. In the past this valve has been neglected so that the air inside the pipes and the rubber sleeve has been pushed into the culm, thus reducing flow considerably and adding to the treatment time. If no bleeding valve exists the culm should be placed much lower than the preservative tank so that some air might escape before treatment begins. 


If older culms with a lower moisture content or culms in the dry season are to be treated it is recommended that they be treated with clean water first to get better saturation and then switch over to the preservative solution tank. A corrugated iron roof sheath or plastic sheath should be placed under the culms to collect liquid spilling from the nodes (Fig. 99). For treatment of single culms a bucket hanging at the end could collect the outflow.

Once pressure has commenced the sap should flow within minutes from the top end and should continue to stream out displaced by the preservative solution (Fig. 100). This will first drip from the vessels at the lower culm part and only later cover the whole cross section. The very first part of clean sap can be withdrawn, later the outflow has to be collected and might be re-used as it contains an increasing amount of preservative. 

The treatment can be stopped either when the volume of the outflow has reached at least 10% of the solid volume of the culm or when the concentration of the outflow corresponds to the original concentration of the treating solution. This can be measured by its density with a hydrometer and also by its pH. A medium-sized culm will release about 1-litre of sap/solution. Treatment time and success depends on the bamboo species, its origin, age and moisture content, culm length and wall thickness, the preservative, the pressure applied and on logistics. In general, culms of 150 mm diameter and 6 m length are treated in 30-50 minutes and those of  9 m length in 60-70 minutes with a pressure of 1.0-1.3 bar (15-20 psi, 1 bar = 15 psi).

It should be taken into account that the determined density of the solution becomes unreliable with time because of the increasing amount of sap and a possible bacterial contamination of the sap. There is little information so far about the effect of sap/sugar on the measurement of density, neither on the precipitation nor the toxicity. The possible interaction of borates with sap has yet to be clarified before possible extended application. The common re-use of the outflow may cause problems, as also its safe disposal, especially in the case of non-boron preservatives. 

The earlier simple bicycle pump by Purushotham et al. (1954) was applied for a pressure of 1.0 to 1.4 bar (15-20psi) and could be transported by bicycle or car for treating the culms directly in the forest for their use in the villages. Such simplified transportable installations are now again in use with motor-driven pumps.

A stationary High Pressure Sap Displacement (HPSD) Method has been developed by FRIM, Malaysia, with CCA preservative (12.1%) and a pressure of 1.4-4.0 bar (20-60psi) (Choo and Gan 1998). Higher pressure requires stronger pipes and clamping systems and may cause leakage from the nodal cuts.

Diffusion Phase

After the culms have been treated they must remain fresh and be stored horizontally above ground under cover for 1-2 weeks. The storage allows the necessary diffusion of the mobile salt components from the preservative filled vessels into the surrounding tissue of parenchyma and fibres. Boron-treated culms should be protected from rain to avoid leaching from the ends and nodes and against sun to minimize cracking during drying.

Main advantages of the modified sap-replacement treatment are the limited need and cost of the technical equipment, the rapid treatment procedure and the complete penetration of the culm with a clean surface, thus avoiding any risks. The solution can be recycled for a certain time, although further investigations are needed. Care must be taken regarding spillage and the safe disposal of residues as well as a possible leaching of boron treated culms in soil contact. As the sap-replacement process has been applied in several countries, some modifications have been developed for simplification and efficiency such as the system applied by Cusack (1999) with a standing treatment tank made from heavy plastic (Fig. 101).

A detailed description of the process and its modifications has been given by the EBF (1994); González and Gutiérrez (1996); Cusack (1999) an INBAR TOTEM (Rao 2001).

and Liese et al. (1998/2002).