MANAGEMENT OF OPERATION AND OPTIMIZATION OF DESALINATION PLANTS. CEUTA AND MELILLA EXPERIENCES

The International Desalination Association World Congress on Desalination and Water Reuse 2013 / Tianjin, China

REF: IDAWC/TIAN13-084

MANAGEMENT OF OPERATION AND OPTIMIZATION OF DESALINATION

PLANTS. CEUTA AND MELILLA EXPERIENCES

Authors: Mr. Carlos Cordón Ureña, Mr. Javier Arrieta Morales

Presenter: Mr. Javier Arrieta Morales

Engineering Director – Cadagua S.A. – Spain

[email protected]

Abstract

In the last years, RO (Reversed Osmosis) desalination plants have achieved higher levels of water

quality and have reduced CAPEX and OPEX significantly. This achievements are due, among other

factors, to the experiences gathered during the operation of the old plants and the incorporation of the

lessons learned as improvements into the new plant designs.

This paper describes the experience gathered in two specific RO desalination plants located in water-

scarce areas, Ceuta and Melilla (Spain). Both installations are among the first ones that were started up

in Spain and have been working near to a 100 % of their capacity and operation time, providing a large

source of data and experience.

Ceuta SWRO plant was built in 1998 and has an actual production capacity of 22,000 m3

/ day.

Melilla SWRO plant was built in 2007 with a production capacity of 20,200 m3 / day.

At both installations we suffered initial difficulties to achieve the required production, at the required

power consumption. After intense studies and some investments, we manage to solve the problems and

nowadays we perform a compliant and smooth operation and maintenance of the plants.

This paper describes these experiences, with special emphasis on the following:

Redesign of the open intake water installation to improve maintenance

Optimization and increase the flexibility of the pre-treatment system

Reduction in the Cartridge filters loading time

Solution of membrane fouling problems

Improvements on the disinfection procedures

Experiences in improving the energy consumption and energy recovery

Experience with new materials

mailto:[email protected]

The International Desalination Association (IDA)

World Congress on Desalination and Water Reuse


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I. INTRODUCTION)

Ceuta and Melilla are two autonomous cities of Spain located on the north coast of Africa, sharing a

western border with Morocco. Two permanently inhabited Spanish territories in mainland Africa. The

population of both cities are approximately 80.000 inhabitants each.

An arid region and relatively isolated location. The need to ensure a secure, safety and constant source

of drinking water was the motivation to build two of the oldest desalination plants in Spain.

Cadagua was awarded to Design, Build and Operate both plants under PPP contract frameworks, and

although being relatively small plants, they have being an important source of knowledge and a learning

platform to improve our designs and operational procedures.

This paper aims to describe in a practical manner the improvements implemented to minimize the

operational costs while ensuring the production capacity of the plants.

II. INTAKE EXPERIENCE

As we all know, there are 2 fundamental types of sea water intakes:

Open intakes

Beach Well or other subsurface intakes

Open intake are by far the most widely used type of source water collection facilities worldwide,

because they are suitable for all sizes of desalination plants and they are more predictable and reliable in

terms of productivity and performance. The feasibility of subsurface intakes is very site specific and

highly dependent on the project size, the costal aquifer geology and other environmental and socio-

economic factors.

The main disadvantage of an open intake in comparison with beach well intakes is that the quality of the

water is worse as the beach wells, or other types of subsurface intakes, filter the water through the

typically sandy ocean floor.

Focusing on open intakes, which is the case of the Ceuta and Melilla plants, and bearing in mind that the

installation is generally comprised of:

Intake filter or screen

Intake duct or pipe

Intake pumping

We may state that the common problems we face are:

- Blockage of the intake pipe: algae, mussels, sponges and barnacles




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- Reduction of the intake pipe section

- Blockage and problems with the sea water pumps

Figure 1: Blockage of the intake filter: algae, mussels, sponges

All problems arising from the intake installation section involving partial or total shutdowns of the plant

are due to insufficient flow to the plant. The solution to such problems, as has been stated, relies on the

dosage of disinfecting chemical products to avoid depositions of such molluscs and algae. In addition

the plant must be prepared to minimize the shutdown periods whenever the growth of organisms cannot

be fully avoided.

The intake screen design is a square mesh structure connected to the inlet pipe. Cleaning this screen

under water is a very time consuming and complicated task. Therefore the procedure was to fully

replace the complete screen with an spare one. The replacement time was also too long . Thus, we have

thought of maintaining the screen frame structure, and making only removable the sides, as they are less

heavy and easier to replace.

Figure 2: Cube bucket design




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On the other hand, for the case of the intake pipe, the problem was the accumulation of waste and

biological growth along the length of the pipe, where access is impossible. Or mechanical cleaning very

expensive. To deal with this, we design intake pipes with manholes every 50 metres. These manholes

have been retro-fitted on the old intake pipes.

In the next figures we can see the manholes and the place where must be installed:

Figure 3: Manhole for the pipe

Figure 4: Location of manhole

With these 2 improvements in the installation, the down time has been considerably reduced and the risk

of total shut-down has been avoided.




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III. PRE-TREATMENT EXPERIENCES

3.1 Quality of the seawater

Just as discussed in the preceding chapter, one of the major problems of open water intakes arises from

the need for a good pre-treatment to improve the water quality in the plant. In the case of beach wells the

natural filtering media, performs treatment prior to the pre-treatment of the plant. That is why in certain

cases it may not even be necessary to use chemical reagents to improve the efficiency of the pre-

treatment.

At Ceuta and Melilla plants. During storm weather conditions, which on the other hand are quite

common, the turbidity values rises up to 20 NTUs. As a consequence, the sand filters clog rapidly and

there is a need for frequent cleaning. The result being a partial halting of the plant production capacity..

Figure 5: Storms at the sea and clogging up of the sand filters

In order to solve those problems, we have implemented the following solutions in the new Ceuta SWRO

plant:

1) Installation of a double filtering stage (as already done in places where the sea water quality

is bad) to improve the quality of the pretreated water entering the membranes;

2) Reducing the filtering speed in the filters, to maintain the filter run.

3) Installing and running a backup filter when the sea water quality is very bad, in order to be

able to use them while cleaning those that have just filled up;

4) Combining the above solutions.

The second stage filters have been design with the same size as those of the first stage. The piping work

and an appropriate set of valves allow us to choose the best operational procedure according to the

quality of the water and the season of the year. With this flexible design we can operate the pretreatment

under the following scenarios:




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a) Operate the second stage as a second stage.

b) Changing the filter media of the second stage and use all filters as first stage, reducing the

filtering speed, and maximizing the filter run during storm seasons.

c) Changing the filter media of the second stage, leave some as back up filters, and in the case

of a very bad water quality, putting them into operation.

The idea is to decide between b) and c) during the worst season of the year, usually between February

and April, depending on the degree of turbidity of the water. At designing stage it is easy to consider the

necessary set of valves and pipework to provide this flexibility to the plant.

Figure 6: Set of valves to operate in one or two stages

3.2. Corrosion of materials of the sand filters

As we all know, the corrosion caused by sea water in the materials is quite considerable, due to which it

is important to choose materials that resist sea water aggression, as long as they comply with the

mechanical requirements. Until now, internally rubber lined steel water filters have been used for

internal protection. Experience tells us that it is easy for part of the internal protection to come loose,

and cause initial corrosion throughout the whole filter. Due to this, GRP filters have been developed to

resist pressures between 6 and 8 bar. These filters do not suffer neither internal nor external corrosion,

reducing drastically their maintenance costs




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Figure 7: Corrosion of the internal layer of sand filter

3.3 Maintenance time to replace the cartridge filters

Cartridge filters are placed behind the sand filters as a safety measure to prevent particles bigger than 5

μm from reaching the membranes. Thus, their maintenance is crucial for the operation of the plant. The

shutdown times to replace the filter cartridges are important and require to shut down the production

line. For this reasson, it is a good idea to have a base plate to hold an additional set if cartridges. With

this installed in a reserve space of the plant, the cartridge replacement may be performed as a block with

the base plate, thus considerably reducing the replacement times.

Figure 8: Cartridge filter kept in a base plate

IV. EXPERIENCE IN REVERSE OSMOSIS AND ENERGY SAVINGS

The problems we have had at these plants concerning reverse osmosis have been:

Difficulties reaching the total production required by the contract to satisfy the client’s demands

and to cover the depreciation costs.




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This basically happened because the plant shutdown times required to clean the membranes have

exceeded those estimated, and the RO train production has been lower due to biofouling of the

membranes.

On the other hand, as it is known, the specific consumption of the plant (Kwh/m3) is one of the

most important ratios to be controlled, as it represents the main cost of an RO installation.

Among the different consumers of the installation, the high pressure pumping to the RO trains is

the most relevant one.

The solutions we have considered in order to improve these aspects include:

To increase plant production by obtaining better performance from the pump and

reviewing the rest of the installation;

Modifying the chemical treatment.

4.1 Increasing the flow (Case of Melilla desalination plant)

The idea was to perform retro-fitting of the high pressure pumps, that may go from F1 = 20,200 m3/day

to F2 = 21,800 m3/day.

To do so, we went from:

THD1 = 605 m.w.c

Number of membrane vessels = 240

Number of membranes = 1680

We performed the retro-fitting of the pump and went to:

THD2 = 650 m.w.c.

Number of membrane vessels = 264

Number of membranes = 1848

For the rest of the water line:

The filtering speed was adapted (increased), maintaining the same number of filters.

Frequency variator installed on the rest of the pumping sets: sea water, filtered water and product

water.

The investment in the RO trains was of 8 pressure vessels, 56 membrane elements and the retro-fitting

of the pump.

The benefit obtained has been an increased production per RO train of 29 m3/h.

As a result of these measures, we achieved a direct energy saving of 0.34 kwh/m3, the return period of

the investment is over 2 years.




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4.2 Modification of the chemical pre-treatment at the desalination plant

At both the desalination plant of Ceuta as well as that of Melilla, we find the following problems:

Need to wash through with biocide between 1 and 2 times a week.

Chemical cleaning with detergent every 2 to 3 months per RO train.

And in spite of this, the differential pressure in the RO train increases quickly.

We performed several membrane autopsies and observed that there was biological growth on the

membranes surface.

Figure 9: Biofouling in membranes

That growth arose from the type and procedure of chemical pre-treatment; the initial quantities were:

182,250 kg /per annum of sodium hypochlorite continuously

145,152 kg/per annum of sodium metabisulphite continuously

18,200 kg/per annum of anti-scalant

With this dossing procedure, and the amount of organic matter present in the sea water, the sodium

hypochlorite and the metabisulphite were breaking down the organic molecules, making them easily

accessible for the microorganisms, and therefore enhancing the biological growth.

After several studies, we decided to change the disinfection strategy from continuous to intermittent

(shoks) which has proven to be more effective.

Sodium hypochlorite (shock) 91,125 kg/per annum

Sodium metabisulphite (shock) 72,576 kg/per annum

Anti-resistant 13,200 kg/per annum

The effects of this treatment were that the differential pressure did not increase as swiftly and the

necessary time for cleaning was much lower.




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Figure 10: Graph of differential pressure and time between chemical cleanings

On the other hand, the specific consumption of the plant was reduced.

Figure 11: Graph of specific consumption

The saving range was 40% in Chemicals and 10% in energy.




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V. EXPERIENCE WITH NEW MATERIALS

Material corrosion, the high cost of the stainless steel, and the maintenance cost of steel are changing

materials selection criteria in the new contracts. For instance one of the most innovative changes is the

GRP or polyester filters with a very low maintenance cost. The outside of the polyester filter is more

resistant to corrosion than the painted steel filters in locations close to the sea. And with regard to the

inner side, experience says that the ebonite-lined steel can have several corrosion problems when the

ebonite lining comes unstuck from the steel surface .

Figure 12: GRP filters

VI. CONCLUSIONS

The feedback form the operators of the plants to the designers is an advisable practice to minimize the

mistakes and maximize the operability of the plants.

Forecasting problems and implementing elements that give flexibility to the pretreatment is much easier

and cost effective at designing stage.

Implementation of simple and effective maintenance solutions at the intake filters and intake pipe is

crucial to ensure the reliability of the complete plant,

The cost of materials and energy are main factors when discussing the life cycle cost of a desalination

plant. Progress must take place in the development of innovative and cost-effective materials as well as

in the energy recovery field.

MANAGEMENT OF OPERATION AND OPTIMIZATION OF DESALINATION PLANTS. CEUTA AND MELILLA EXPERIENCES

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