Electronic Waste or E-Waste

Have you ever given a thought what happens when you throw or sell your old cell phone or personal computer? Where does it go? And how it affects the atmosphere? Electronic waste or e-waste are those electrical and electronic items that are no longer usable or have been replaced by the new generation version. Computers, cell phones, televisions, refrigerators, air conditioners, DVDs, iPods, copiers, and fax machines are common electronic products, which eventually turn to e-wastes. According to researchers nearly 75 percent of old electronic products are dumped in storage because of the uncertainty of how to manage these materials. Although many of these materials can be reused, refurbished, or recycled but unfortunately they are not, thus making the electronic discards as one of the fastest growing segments of world’s waste stream.

With the growth of technology many new companies are stepping in the market with new and/or new versions of the products like laptops, computers, cell phones, televisions, music players etc. Disposal of computers and cell phones is the major segment of electronic waste. As technology improves the lifespan for electronic devices such as computers and cell phones becomes shorter. In developed countries these electronics have an average life span of two years. In the Unites States there are more than 300 million obsolete computers. Although most electronic devices that are thrown away still have parts that are reusable.

 Current Scenario

Rapid product innovations and replacement especially in IT sector combined with migration from analogue to digital techniques have together shifted us to the electronic world. The growing economies of the world have given way to lower prices for many electronic goods which has in turn increased global demand for these goods. Increasing production of new electronic goods put the old electronic products into store labeling them ‘e-waste’.

People are upgrading their computers, cell phones, televisions, audio players and printers more frequently than ever before. Presently cell phones and computers are causing the biggest problem because they are replaced most often. That is the number of discarded electrical and electronic waste is piling up to millions. Electronics waste now makes up five percent of total municipal solid waste worldwide which is almost equal to the waste of all plastic packaging material. Not only developed countries but the developing countries also have its share in the production of E-Waste.  According to the reports, Asia discards an estimated 12 million tons of E-Waste each year. While the electronic waste stream has increased dramatically in the last 10 years; efforts to regulate or recycle them are being developed at a much slower pace.

A recent report by United Nations predicts that by 2020 e-waste from old computers in South Africa and China will have jumped by 200–400 % and by 500 % in India compared to 2007 levels. It also states that by 2020 e-waste from discarded cell phones will be increased to 7 times than 2007 in China and 18 times in India. This report also mentions that in the United States more than 150 million mobiles and pagers were sold in 2008, up from 90 million five years before, and globally more than 1 billion mobile phones were sold in 2007, up from 896 million in 2006. The UN report estimates that countries like Senegal and Uganda can expect e-waste flows from personal computers alone to increase 4 to 8-fold by 2020.

 Why it poses a threat?

Electrical and electronic equipments contain several hazardous materials which are injurious to human health and the environment if not managed to recycle properly. Most of the electronic parts contain toxic chemicals. When dead computers are placed in landfills, burned, or improperly recycled, toxic chemicals presented in the electronic parts are released into the air, water and soil. The most dangerous chemical in technologic devices is lead which is used in soldering of circuit boards. It causes problems to the human body, nervous system, kidneys, liver and child development. It can cause retardation and high blood pressure. Some naturally occurring substances are harmless in nature but their use in the manufacture of electronic equipment often results in compounds which are hazardous, for example chromium becomes chromium VI.  The following table presents the information about the toxic substances presented in e-waste.

According to current trend e-waste travel large distances to developing countries, where rudimentary techniques or the backyard methods are implied to extract precious metals or recycle parts for further use. But this disposal is hazardous to health of the poorly protected workers and communities in the vicinity. E waste amounts to 40 million tons per annum, which is enough to fill a line of dump trucks stretching half way around the globe. This illustrates the need for a holistic view to be taken in analyzing the e waste situation for working out possible solutions to tackle the problem well.

 Proposed Solution – Recycling

Considering the threat seriously we need to have ways to recycle the e-waste generated to maximize resource recovery and minimize potential harm to humans and the environment. It needs a much more speculated method for disposal as it contains heavy metals, rare earth metals, precious metals and some hazardous metals as compared to the disposal of household waste. The discarded electronics contain materials which might be hazardous and unhealthy if disposed of improperly. This is the era when the technological advancements are accompanied by thoughts and ideas to initiate the campaign to protect and save the earth and the environment.  So we need to strategize the disposal of e-waste and lay emphasis on refurbishing it which will protect the environment along with the economic benefits.

E-waste management includes the three r’s of waste management - reduce, reuse and recycle. Maximize the reuse of electrical items to the safer limits when the radiations or ill effects are under control. The reuse will diminish the demand of newer products and has several other social and environmental benefits.  Recycle the discarded electronics to re-utilize the various elements used in manufacturing. Recycling the e-waste we can replenish about 15% of the earth minerals and metals we need and can be reused to satisfy the increasing demands of electronics. For example 80% of indium demand is for the LCD screens. Recycling the broken and unusable screens can save the earth’s mineral.

Today e-waste management and recycling is not only a noble cause but also a consolidating business. A whole market is available to make products of the recycled plastics and metals recovered from the electronics. So it can be rightly said that it’s just not an emergency to curb the problem of e-waste with a heavy hand but also an opportunity for business generating decent employment. Along with this it is going to reduce the green house gas emission, save many precious and rare earth metals. If dealt properly the e-challenge can be successfully converted to e-opportunity. To quote a few examples the audiovisual components, televisions, stereo-equipments, mobile phones and other handheld devices, and computers contain valuable elements and substances suitable for reclamation, including lead, copper and gold. Almost all electronic items contain lead and tin (as solder) and copper (as wire and printed circuit board tracks), though the usage of lead free solder is in now. Elements and compounds found in general in the electronics include epoxy resins, fiberglass, polyvinyl chlorides, thermosetting plastics, lead, tin, copper, silicon, beryllium, carbon, iron and aluminum, americium, antimony, arsenic, barium, bismuth, boron, cobalt, europium, gallium, germanium, gold, indium, lithium, manganese, nickel, niobium, palladium, platinum, rhodium, ruthenium, selenium, silver, tantalum, terbium, thorium, titanium, vanadium, and yttrium. As we know many of these elements are precious but have potential danger and need to be dealt very safely.

 Recycling of E-Waste takes place in three major steps which are depicted here.

1. Detoxication

To ensure safe disposal or recycle of E-Waste, detoxication process takes place for the electronic materials. Detoxication is the process of removing critical components from the electronic waste in order to avoid contamination with toxic substances during the downstream processes. Critical components include lead glass from CRT screens, CFC gases from refrigerators, light bulbs and batteries.

2. Shredding

In the next step electronic materials are broken into pieces to obtain concentrates of recyclable materials in a dedicated fraction and also to further separate hazardous materials. The mechanical processing plants where shredding takes place includes shredders, crushing units, magnetic and eddy current and air-separators. The emitted gases are filtered and residues are treated to minimize environmental impact.

 

3. Refining

The third step of e-waste recycling is refining of the shredded materials to obtain reusable components. Refining of resources in e-waste is possible and the technical solutions exist to get back raw with minimal environmental impact. Most of the components need to be refined or conditioned before they can be sold as secondary raw materials or to be disposed of in a final disposal site.

There are some more techniques that are employed to destroy or recycle the E-waste, enlightened in the following section of the article.

Incineration:

It is a process to destroy the waste by burning. As there are toxic materials present in the e-waste there may be a risk of generating and dispersing contaminants and hazardous substances. The gases released during the burning and the residue ash is often toxic especially when it is employed without prior treatment or sophisticated flue gas purification. Metals like copper, which is presented in printed circuit boards and cables abundantly, works as a catalyst for dioxin formation when flame retardants are incinerated. These brominated flame retardants can lead to the generation of extremely toxic polybrominated dioxins (PBDDs) and furans (PBDFs) when exposed to low temperature (600-800°C). PVC, which can be found in e-waste in considerable amount is highly corrosive when burnt and also forms the dioxins which are dangerous for both live creatures and atmosphere.

Open Burning

Generally e-waste is burnt in the open fire but since it is burnt at low temperatures, it releases relatively more pollutants in the atmosphere than incineration process. Inhalation of open burning emissions may cause serious health problems such as asthma attacks, coughing, chest pain, eye irritation, etc. If open fires burn with a lack of oxygen, it forms carbon monoxide, which poisons the blood when inhaled. In the burning of e-waste chronic emissions are released which may lead to diseases like emphysema and cancer. The PVC releases hydrogen chloride when it is burnt, forms hydrogen chloride acid reacting with water. It can lead to corrosion of lung tissues and other respiratory complications.

Landfilling:

It is the most widely used method for waste disposal. If the landfills are leaked, e-waste as it contains heavy metals and toxic substances can contaminate ground and water resources. Although landfills are sealed to prevent toxins from entering the ground but with the time they may have chance to be leaked and thus pose a much greater danger of releasing hazardous emissions. Besides leaking, vaporization is another concern in landfills. Significant impacts from land filling could be reduced by separating dangerous materials from e-waste and by land filling only those fractions for which no further recycling is possible. Also it should be ensured that the whole process respect environmentally sound technical standards.

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                             CMOS Image Sensors

Advent of CMOS technology in eighties led to the phenomenal growth in semiconductor industry. Transistors have become smaller, faster, consume less power, and are cheaper to manufacture. It is CMOS technology which has enabled very high integration on the chips leading to modern high performance, miniaturized integrated circuits.

Apart from the valuable contribution in miniaturization of integrated circuits, CMOS technology found applications in sensing applications. CMOS technology has been adopted to design sensors, especially in the field of imaging. Due to the wide usage of CMOS based image sensors, CMOS sensors are often considered to be a synonym of CMOS based image sensors and have emerged as a competitor to CCD based image sensors.

Until recently, Charge Coupled Devices (CCDs) dominated most of the image sensing systems, i.e., cameras, camcorders, etc. CCDs have been in use in astronomical cameras, video camcorders and scanners. However of late, CMOS Imaging have emerged as an alternative to CCD imagers and it also offers better features.

Subsequent sections will discuss both CCD and CMOS sensor based imagers, their pros and cons, and also their applications. Further, other applications of CMOS technology in the field of sensing will be discussed.

CMOS Vs CCD

Invention of CCD marked the end of vacuum tube imagers used in television cameras as it overcame the disadvantages of vacuum tubes like chronic picture artifacts as lag and burn-in, fragility of large glass tubes or the sensitivity to shock, vibration and electromagnetic radiation, painstaking periodic alignment of tubes, etc. It also marked the beginning of a new era in imaging systems and for decades, it enjoyed quality advantages over the rival CMOS sensors. Wherever image quality was paramount, CCDs were preferred, CMOS were used mainly in applications where small size and low power were prime requirements.

With the technological development in CMOS technology, gap between CCD and CMOS sensors has narrowed; CMOS sensors can also achieve competitive quality. Choice amongst CCD and CMOS sensors has become increasingly difficult.

Both CCD and CMOS image sensors use large arrays of thousands (sometimes millions) of photo-sites, commonly called pixels. Both carry out same steps.

1. Light-to-charge conversion

Incident light is directed by the microlens (a tiny lens placed over the pixel to increase its effective size and thereby fill factor) onto the photo-sensitive area of each pixel where it is converted into electrons that collect in a semiconductor "bucket."

The bigger the pixel, the more light it can collect. Thus, big pixel sensors work best under low-light conditions. For the same number of pixels, bigger pixels results in bigger chip, this means higher cost. Conversely, smaller pixels enable smaller chip sizes and lower chip prices, as well as lower lens costs. But there are limitations on pixel size reduction. Smaller pixels are less sensitive to light, the optics required to resolve the pixels becomes expensive and requires expensive fabrication possesses.

2. Charge accumulation

As more light enters, more electrons accumulate into the bucket.

3. Transfer

Accumulated charge must be transferred to the signal conditioning and processing circuitry.

4. Charge-to-voltage conversion

The accumulated charge must be output as the voltage signal.

5. Amplification

Voltage signal is then amplified before it is fed to the camera circuitry.

Both CMOS and CCD perform all these tasks; however the aspect in which they differ is the order of execution of these tasks.

BRIEF ON CCD TECHNOLOGY

CCDs were first invented in 1969 as a way to store data using bubble memory. In 1974, the first imaging CCD was produced by Fairchild Electronics with a format of 100x100 pixels.

CCD imager consists of two main parts: color filter and pixel array

•           Color filter

Micro-lenses funnel light onto the photo-sensitive part of each pixel. On their way, the photons pass through a color filter array. The mosaic of these tiny filters captures color information. Color filters enable separate measurement of the red (R), green (G) and blue (B) photons. Color filter filters out wavelengths of unwanted colors and allows only specific colors of light to pass through a pixel sensor. For this purpose, each pixel is covered with a red, green and a blue filter according to a specific pattern, like the Bayer CFA pattern.

                     

Bayer filter uses the sub-mosaic 2x 2 patterns with one red, one blue and two green filters. As human’s eye has greater sensitivity for green light, two green filters are used.

•           Pixel Array

The pixel array functions on the principle of the photoelectric effect and pixel sensors are responsible for capturing the intensity of the light passing through. The light intensity data is combined before being converted into an analog voltage signal, which is outputted to an external circuit board to be further processed.

After conversion of incident light into electrons, electron charge is accumulated in the same way as bucket stores water. The pixel charges are read using vertical and horizontal shift registers which act as charge carriers.            

CMOS SENSORS

A typical CMOS is an integrated circuit with an array of pixel sensors. In contrast to CCD, each pixel sensor in CMOS sensors contains its own light sensor, an amplifier and a pixel select switch. An analog-to-digital converter and other components critical to the operation of the pixel sensors are located on the CMOS sensor.

The CMOS sensor contains four main parts: the color filters, the pixel array, the digital controller, and the analog to digital convertor.

•           Color Filter

Color filter is the same as was described in CCD based imager.

•           Pixel Array

As in the case of CCD, function of the pixel array is to capture the intensity of the light passing through. Each pixel sensor converts the sensitivity of the incoming light to the voltage signal which is then fed to ADC for further processing

           

There are two types of architectures of Pixel sensors: Passive Pixel Sensor (PPS) & Active Pixel Sensors (APS).

In Passive Pixel sensors, only one photo-detector (without any local amplifier) per pixel is used, whereas in Active Pixel sensors, 3-4 transistors per pixel are used. Passive Pixel sensors have smaller pixels and large fill factor but they are slow and have low SNR. On the other hand, active pixel sensors are fast, have good SNRs but larger pixels and low fill factor.

However, due to advancement of CMOS technology down to nm, pixel size/fill factor is no longer a big issue and APS is the technology which is preferred and used in most devices.

•           ADC

The ADC takes the analog voltage signals from the pixel sensor array and converts them into a digital signal.

•         Digital Controller

The digital controller governs the functioning of the the CMOS sensor; it controls the pixel array, ensures synchronism between all pixels, etc.

Operation of CMOS Sensors

a)         Pixel sensor acts like a charge bucket; accumulates electron charges the same way as water bucket stores water

b)         Charge is converted to voltage & amplified at the pixel.

c)         Individual CMOS microwire carry voltage from one pixel at a time, controlled by the pixel select switch

d)         To output video signal, following steps are followed

1.         All pixel select switches are turned ON. This outputs voltage of each pixel to column circuit.

2.         Column select switches are turned ON from left to right. In this way, signal voltages of each pixel in the same row are output in order.

3.         This is repeated for all rows from the top to the bottom in order, signal voltages of all pixels can be output from the top-left corner to the bottom-right corner of the image sensor.

e)         These signal voltages are output to the signal processor of the camera.

CMOS SENSOR TYPES

Difference between types of CMOS sensors is generally due to the number of transistors (affecting fill factor) that are present for each pixel. A portion of the pixel sensor that is actually sensitive to light is called fill factor.

a)         Rolling Shutter type

This has got limited number of transistors and therefore has a high fill factor. However, lines of pixels are exposed at different times and therefore, movement in the target gives a distorted image.

b)         Global Shutter type

The number of transistors is high in this case resulting in a low fill factor. But, all the pixels are exposed at a time and thus the movement artifacts associated with rolling shutter type sensors are removed.

CCD AND CMOS SENSORS: PROS AND CONS

1.         Fabrication Process

CCD sensors use specialized fabrication that uses dedicated and costly manufacturing processes, whereas CMOS sensors rely on standard CMOS technology (used for IC fabrication like microprocessors, memory, etc.). As CMOS sensors can also integrate required electronics on the same chip, CMOS sensors results in compact and cost effective system

2.         Dynamic Range

Dynamic range of CCD is roughly twice as that of CMOS sensor. This implies that if better colour depth is required, CCDs are likely to offer better results. On the other hand, CMOS are marginally more photosensitive.

3.         Power Consumption

CMOS cameras have lower power consumption than CCDs but other CMOS circuitry may require more power. Low end CMOS sensors have low power requirements, but high speed CMOS cameras typically require more power than CCDs.

4.         Noise

Two types of noise affect sensors’ performance: Temporal Noise and Fixed pattern noise. Fixed pattern Noise is more in CMOS, compared to CCDs because charge is converted to voltage at each pixel as compared to single point charge-voltage conversion in CCDs. In terms of temporal noise, CMOS sensors are better as the bandwidth of amplifiers at each pixel is lower than the output amplifier in case of CCD.

5.         Image Quality

Due to poor fill factor of CMOS, photosensitivity of CMOS sensors is poor in low light conditions.

6.         Uniformity of response

CCDs use single amplifier for all pixels and CMOS use separate amplifiers for each pixel. Pixel-to-pixel amplification differences lead to non-uniformity. Response of CCDs is pretty uniform.

7.         Speed

CMOS sensors have higher speed due to the fact that it uses active pixels and ADCs on same       chip leading to lesser propagation delays.

8.         Readout area

CMOS sensors allow any region or even multiple regions to be read off the sensor. CCDs are limited by vertical scan read out

9.         Smart functions

With the integration of signal processing circuitry on the CMOS sensor chip, functions like auto gain control, auto exposure control etc., anti-jitter, image compression, color encoding, motion tracking, etc. can be incorporated on-chip.

10.       Overexposure effect

Overexposure can cause smearing around over-exposed pixels. Smearing is caused by spilling of charge into the shift register. Due to absence of shift registers in CMOS sensors, they are immune to this effect.

OTHER CMOS SENSORS

•           CMOS Humidity & Temperature Sensors

Swiss-based Sensirion AG has introduced an integrated, digital humidity and temperature sensors using CMOS "micro-machined" chip technology. SHT11 is a single  chip RH and temperature sensing module with a calibrated digital output fabricated  using CMOS technology.

           

Conventional humidity sensors determine relative air humidity using capacitive technology. However they suffer from poor long-term stability and complicated calibration. In addition, they require additional circuitry to convert analogue output to be interfaced with microprocessors, etc.

SHT11 uses a micro-machined finger electrode system with protective and polymer cover layers forming the capacitance for the sensor chip as well as simultaneously protecting the sensor from interference. The sensor chip can be interfaced directly to any microprocessor system by means of the digital 2-wire interface.

As the temperature sensor and the humidity sensor are integrated in a single unit, it removes measurement errors due to temperature gradients between the two sensing elements. The sensor offers high noise immunity, good stability, short response times, high precision, low power consumption, and has a small footprint

•           3D Imaging Sensor

DepthSense™ sensor is a patented CMOS chip from SoftKinetic DepthSense for 3D Imaging. It uses time-of-flight principle and provides a direct way for acquiring 3D information of objects enabling new applications such as gesture recognition. Such sensors are being integrated in cameras.

KODAK 9000 3D Panoramic System is another 3D imaging system for dental applications. With an impressive focused-field 3D, it can generate different types of facial images.

•           CMOS X-ray Detectors

Flat panel CMOS X-ray detectors from Dexela are based on innovative crystalline Silicon based CMOS sensor design offering unprecedented speed and superior image      quality. The     detectors are suitable for a range of applications including mammography and tomosynthesis, angiographic imaging, bone densitometry, dental CBCT, scientific instrumentation and NDTs.

           

The major advantages of the technology are: high frame-rate, low noise, high reliability, absence of image lag and high spatial resolution. The clinical benefits are lower radiation dose to the patient combined with superior image quality when compared with flat panels based on amorphous Silicon sensor technology. Dexela uses the flexibility, speed and low noise of CMOS technology to create detectors that are flexible, faster, more sensitive, higher resolution and more stable than TFT based flat panel detectors

The main components of this detector are: CMOS image sensor, scintillator, control electronics, readout electronics and communications devices.

•           CMOS Microhotplates

Cambridge CMOS Sensors provides innovative MEMS high temperature microhotplate technology for gas sensing, flow sensing and lab-on-a-chip applications. Technology uses standard CMOS process that enables high volume, low cost and low power sensor-on-a-chip solutions

•           CMOS sensor for fluid flow

Burkert Fluid Control Systems uses CMOS technology for mass flow rate measurement of the gas by measuring the temperature differential.

It uses a Si chip in one place with an exposed diaphragm on the walls of the flow channel. A resistor is connected to the diaphragm; two temperature sensors are installed at upstream and downstream of it.

If the heating resistor is supplied with an excitation voltage, the voltage differential of the temperature sensors provides mass flow of the gas flowing through the channel.

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