Continuous non-mechanical thickening and mixing system utilising a South African developed plastic cell matrix

 

Introduction

An inexpensive honeycomb plastic cell matrix has been developed in South Africa. Hitherto use of the cell matrix has been limited to civil engineering applications. However, the matrix can serve as a core in continuous non-mechanical thickening or mixing processes resulting in improved efficiencies and cost savings. A pilot plant is currently under construction enabling full scale studies.

System Overview

The system comprises a "honeycomb" core installed in a rectangular or circular vessel. The honeycomb core is fabricated by joining plastic sheeting and may be up to 4 metres deep The core is equipped with apertures positioned so that the pulp fed into the apparatus is forced to travel a circuitous route to the discharge point (see figure 1). The core can be installed in a variety of existing vessels or else a suitable pond may be constructed. The core may be a cube or may be cylindrical depending on the vessel.

The core would normally be fabricated from high density polyethylene film but can be fabricated from a variety of different plastics that are inert to most chemicals and do not rust or rot. Operating temperatures can range from -5C to +80C.

The core is collapsible to 10% of its volume and is lightweight - 10kg per m when opened. It can be removed or temporarily collapsed in situ for cleaning.

 

How does the system work ?

GENERAL

The fluid is caused to flow "down" in the first row of cells and up in the neighbouring row of cells. The process is repeated with multiple alternating rows of "down" and "up" cells. This increases residence time for a given flow rate.

If the flow rate is controlled so that flow is mostly laminar, the honeycomb core offers a system for gravity separation without the cost of mechanical separators and with a far smaller space requirement (land use) than conventional settlement ponds. However, the conventional horizontal flow is now in the vertical plane and is discussed below.

Qualitative tests performed are encouraging. A cell pack 600 mm deep and 2 metres long fabricated with cells 70 mm x 70 mm was used as a clarifier. Input contained 34% manure solids while the output had been reduced to only 0.1% solids. Residence time was only a few minutes. A batch of manure sludge was simply poured into the system. A full scale trial is currently under construction for quantitative evaluation and is discussed later.

The honeycomb can also be equipped with internal baffles and used at non-laminar flow rates as a static mixer.

General advantages

General advantages are :

The cell core is inexpensive. Retail price (as at year 2000) varies from R150 per m for the XX large size to R750 per m for the smallest size. Dimensions of XX large cells are 600 mm x 600 mm and the dimensions of the small cells are 150 mm x 150 mm. A 10 metre long section of settlement pond filled with small cells 4 metres deep would result in a flow path of (10 / 0.15) x 4 = 266 metres.

THICKENING

With conventional continuous thickeners , the feed of the thin pulp is at the centre of a cylindrical tank, the overflow of clear liquid at the periphery of the tank and the discharge of the thickened pulp at the bottom of the tank. Gravitational settlement takes place over the whole area of the tank and four zones generally develop :

From top to bottom these are :

  1. Clear water or solution
  2. Pulp of feed consistency
  3. Pulp in transition from B to D
  4. Pulp in compression

Equivalent zones can be expected in each individual cell column of the honeycomb cell thickener. However only the upper zones are transferred to the downstream cell and it is observed that by far the greater bulk of solids is quickly settled out and lie under the first few cell rows. Thereafter as one proceeds down the length of the tank the particle size of the deposited solids gets smaller and settling time is longer.

The supernatant liquid in successive columns is also graded in clarity. This can have practical use. In a brewery, for example, partially clarified water could be tapped off and used for flushing down floors, purer water for first stage bottle washing and so on, using different grades of water for different purposes.

 

Thickening mechanisms

Downward flow in a cell column assists transport the particle to the bottom of the tank. Provided that the flow is laminar the settlement velocity will therefore be higher than the free-settling velocity resulting from the balance between gravitational acceleration and frictional drag of the fluid.

Upward flow in a column has a different effect. In this column the frictional drag assists gravity slow down the velocity of the particle. Particles will settle less quickly than in a "down" column but efficiency of separation will be improved and fewer particles will be transported to the next downstream column.

Turbulence

It is critical that turbulence is not allowed to develop in the chamber where a "down" column joins an "up" column. Such turbulence could entrain particles that had been previously separated. Turbulence can be minimised by ensuring that the apertures between adjoining cell columns are sufficiently large and by limiting the volumetric flow. As fine particles are more likely to be entrained than coarse ones it should be arranged in the design of the core that the space above the solids in downstream chambers are larger than upstream ones. This can be achieved by having larger apertures between cells or by using different sized cells for different sections of the tank.

FLOCCULATION AND DEFLOCCULATION

The dividing chambers between cell rows serve as appropriate locations for introducing flocculants.

FILTRATION AND PERCOLLATION ADSORPTION

The cell core can be filled with a suitable filler material such as sand for filtration or most commercially important granular adsorbents.

FLOTATION

Air can be introduced into one or more rows of bottom chambers using a manifold to distribute the air across the width of the tank. Flotation concentrates can be drawn off using the upper chambers.

STATIC MIXER

The cell pack can be fitted with extra baffles within the cells - a full scale trial section has been fabricated but has not as yet been subjected to any tests.

 

 

The baffles will ensure maximum turbulence as illustrated in figure 3.

Additional components to be mixed can be added at chambers marked "A".

Air or other gas can be added in chambers marked "B"

Potential applications

The apparatus has application as a thickener or mixer in the following applications.

The thickener trial

A trial is presently under construction at a large dairy farm in Bethel.

The thickener is an essential component to the overall project - a low-cost method of generating electricity from farm animal waste using a geocell structure. The thickener will process 400 m of manure sludge containing 2% solids per day. A conventional thickener could have been used but this would increase costs and the overall project would not then be viable.

Some detail of the methane generating system is included for completeness and the reader's interest although the methane generator is not the topic of this paper.

DESCRIPTION OF THE PROCESS

The digester comprise of a specially designed geocell / concrete lined tank (90m x 20m x 10m) fed by a thickener equipped with a geocell core. The digester is located below the soil surface and is covered with a plastic solar heating lid.

Liquid animal waste (2%), is flushed from the animal production units to the thickener, where the solids are separated by the geocell internals to yield a 12 % mixture.

The thickened pulp is then routed to the methane tank which is maintained at 35C, optimum for methane production. The clear liquid is recycled to flush the production unit.

After a residence period of approximately 30 days at a temperature of 35C, methane is produced in the separated solids layer.

The methane is converted to electricity (120KVA/day) using internal combustion engines to power 380V generators.

The balance of the solids is converted into compost, while excess water is pumped on crops as a fertiliser after passing through algae ponds and fish ponds.

CURRENT FEEDLOT PRACTICES IN SOUTH AFRICA

Where big feed lots are in production in South Africa the current practice is mainly to simply move the manure to one side and leave it in heaps. This practice leads to contamination of groundwater and rivers and is in the process of being legislated against.

CONVENTIONAL TREATMENT PROCESSES AVAILABLE

There are two main systems in use for treatment of agricultural animal waste.

  1. settlement ponds, (b) conventional methane digesters

The geocell system consists of a combination of these systems with innovations to improve efficiency and to reduce costs in each, for example an impervious waterproof liner is sandwiched in the geocell filled with armouring concrete to prevent pollution of ground water. In addition the geocell system produces end products which have value. The end products are (a) electricity, (b) liquid fertiliser, (c) treated water (for discharge or for re-circulation), (d) compost from the sludge, (e) spirulina algae (70% protein) for fodder, and . (f) talapia and silver carp fish protein.

DISADVANTAGES OF CONVENTIONAL SETTLEMENT PONDS

The system has to be operated in duplicate. When the first pond becomes full with sludge the effluent is switched to the tandem system. The pond is allowed to dry out and is then scraped clean using mechanical equipment (a loader).

Cost of construction of the concrete tanks is high ( R700,000)

Occupies large areas of land

Consumes large quantities of water and has high evaporation losses

Is very smelly - environmentally unacceptable.

Trade restrictions with overseas export markets

Generates no income

Summary

A low-cost geocell developed in South Africa for civil engineering applications can also be manufactured with a thickness of up to 4 metres. This deep honeycomb matrix can serve as an inexpensive core for continuous non-mechanical thickening and mixing systems.

Thickening and mixing are well understood but the geocell system introduces some novelties. In particular the flow path is in the vertical plane and not the customary horizontal plane. Qualitative tests have given favourable indications of the efficiency of the system. A full scale pilot plant thickener is incorporated as an essential component of a pilot project to convert 400 m per day of dairy farm waste to electrical power. The pilot methane plant (incorporating the thickener) is currently under construction (January 2000). This is a major project and will serve to quantify the indicative tests.

Should the trial prove that the geocell method is both viable and cost effective then there is a wealth of applications where the technique can be applied in agriculture and industry.