Wastewater Sampling and Analysis

Wastewater treatment plant relies heavily on data and information collected from test results in order to perform to its expectations meeting not only quality but performing up to its maximum capacity. Nevertheless in order to produce accurate data, a collection system needs to be developed that focused on the method to obtain a representative sample. A sampling plan need to be scheduled accordingly in anticipation towards the change on treatment demand including increased capacity and pollutant levels. Thus the wastewater plant operator must be aware of the action plan that needs to be taken upon realizing that there is a change of conditions.

Usually the incoming influent streams have a relatively standard and constant quality except that the level of suspended solids can vary depending on how the sampling procedure is being carried out. For example, along a distribution pipe, the most ideal location to collect the sample would be from a protruding sampling line that extends into the flow path of the stream rather than tapping a single line coming from the wall of the pipe. The reason for doing that is simple because solids tend to accumulate on the surface on the wall and thus a sampling point that draws wastewater from that location would mean having a test sample having higher than normal solid content that would not be representative of the actual scenario at all. Similarly if you have an open distribution weir or flumes, the way on how the sample is obtained can also influence the level of floating material present in the wastewater results. Equally important are how the samples are drawn and taken and whether the containers like jars are properly rinsed and flushed beforehand.

In order to understand when and where to obtain sample requires that the engineer conduct a study and analyzing the process control on a whole. For instance, certain locations might not be suitable due to the reason that mixing is incomplete and that it might contain higher proportion of wastewater coming from other steams. Thus the person must be knowledgeable of the sewer system so that the closest representative sample can be collected. Sometimes the sewer pipe may not be completely filled leading to conditions whereby the upper layer will have higher level of floated materials and the lower portion having higher than expected sediment accumulation. These criteria have to be carefully considered and analyzed before the person decides on whether the collected sample can provide the closest representation of the actual scenario.

Usually before a sampling schedule is set in place, it would feasible to first conduct a stream profile analysis in order to assess the variability. One of the methods to achieve this is to install an automatic monitoring system such as probe to collect data such as pH, conductivity, temperature so that a graph can be plotted out. Based on this data, fluctuations that occur during the specific period of time will then determine the sampling intervals needed. Nevertheless if the results are relatively constant throughout the whole study period, then sampling frequency can obviously be reduced. Even with the absence of continuous monitoring device, hourly manual sampling can also be performed in order to determine the future collection schedule by estimating the proportion of flow rate past the individual sampling point. The measured volume based the on flowmeter reading can then be used to calculate and determine the composite.

There are different types of automatic samplers available to suit the characteristics and nature of the wastewater. Some sampling device are meant to cater for the need to obtain sample from the surface while there are others specially made for drawing from different depths of the tank. There are even designs made using on different shapes for placement in a calibrated flume so that the sample amount collected is proportional to the overall flow. Once the sample has been collected, the first observation is very crucial because a fresh sample can give a better representation when it comes to odor, physical appearance such turbidity, floated material and also color. There is a long list of test methods for different parameters that can be used to determine the characteristics of the waterwater and usually the need to conduct certain tests are determined mainly by regulatory requirement or based on the expected characteristics of the waterwater. Nowadays the main challenge is not only the refine the accuracy and speed of testing but what is more important is how some of the wastewater constituents present in lower concentrations can be measured.

Measuring Wastewater Flows

In a disk type flowmeter, the displacement of the disk is directly proportional to the amount or volume of water passing through. Similarly a turbine type flowmeter is also constructed based on the similar design principle whereby the flow of water will cause the multiblade rotor to turn proportionately. Based on this criterion, wastewater flows can be measured accurately in order to provide information and data about the incoming water so that the operators and technician can adjust the downstream processes accordingly.

There is also another different method based on pressure differential. Since water cannot be compressed, the amount passing through a conduit in a period of time can be used to represent the flow of through another conduit and it does not get affected by the line size restrictions. Since volume remains constant, the flow velocity will increase when it passes through a smaller orifice or opening and based on this design principle using differential pressure on both sides of the device, somehow an accurate flow measurement can be obtained. Thus with orifice providing the restriction leading to head loss at the other side, a formula can be used to calculate the corresponding flow per minute. However, the weakness when it comes to using orifice is that it can causes permanent head loss meaning that energy is wasted and probably the wastewater will not end up reaching its final destination. With that in mind, venturi tubes were developed so that the loss can be reduced significantly. Flow nozzles also function based on the same working principle but its effectiveness lies between venturi tubes and thin-plate orifice.

A rotameter is also widely installed in the wastewater treatment processing and disposal plant. It is also a flow measuring device which is operated and built based on the pressure differential criteria. What it has is a metal bob placed in the transparent tube which has a taped inner bore so that movement of water will create pressure difference which will support the weight of the bob. The floating position will give an estimation of flow of wastewater by comparing it with the level shown on the graduated vertical indicator on the rotameter tube. This together with the disk type flowmeter as both considered inline measuring devices because the equipment will be present in contact with the wastewater in order to get a reading. However, since the characteristics of the incoming wastewater still contains considerable amount of solids, their application and usefulness were degraded as solids tend to choke and block the measuring equipment. With that in mind, external flow devices were developed.

There are basically two different types of flow measuring device that that external element which are designed to be located on the pipe surface without obstructing the flow of liquid. The magnetic flowmeter is based on principle of operation using Faraday’s law whereby a conductor moving through a magnetic field using electrically energized coil will produce voltage proportionate to the velocity of the passing liquid. The voltage information will be relayed via a transmitter to a meter and using calibrated graph defined earlier, it will converts the signal to flow reading. Similarly for an ultrasonic flow measurement, a sensing element and a transmitter will be strapped onto the surface of the pipe and the waves projected will be received by at the other end. There are two techniques on how the flow will be determined. The first one is based on the time travel difference whereby there are two transmitters in which the beam projected by the first unit will move downsteam while the other one will move upstream and with the time difference for it to travel towards the receiver; this will be taken as proportionate to the flow. However, disposal wastewater with high concentration of suspended solids will interfere with the accuracy of the reading. The Doppler technique on the other hand, is more suitable to be used on moving liquids that has high amount of solids and bubbles present in it. The reason why it can produce such accurate reading is because the projected ultrasonic beam relies on the echo back reflection when it hit the solid particles and the change in frequency will be calculated as proportionate to the flow.

Water Contaminants & Treatment Methods

The introduction and source of contaminant into general drinking water supply is mostly due to industrial activities. Contaminants can be divided into two types, one is a dissolved contaminant and another is a non-soluble constituents. The soluble materials can be grouped into five different classifications based on its individual concentration levels. Let’s move on to study each of these in details.

Class 1 – Major constituents with dissolved solids exceeding the 5ppm concentration
Bicarbonate is usually introduced to the water supply caused by bacterial biological activity producing carbon dioxide that goes on to dissolve in the water. Apart from that, the human population also is partly to blame as we also cause the alkalinity to increase through the use of industrial chemicals. Normally alkalinity level is supposed to maintain at the lower range level but there are also instances whereby industrial activity such as neutralization of chemicals used in food applications and textile treatment will leave excess caustic leaching into the water systems. Calcium is the major constituents that lead to increased hardness level. The mineral is present in many forms and the level rarely exceeds 500ppm but there are also instances whereby increased level can be caused by dumping of chemicals coming from the softening operations. Usually it can be dealt with by introducing lime softening agent by changing it to the form of calcium carbonate. Chloride is very common and since it is very soluble in water, the level can vary accordingly depending on the source on where it comes from. For instance, the concentration and chloride level in sewage is much lower compared to the water source located closer to the sea.

Class 2 – Secondary constituents with the concentration between 1 to 10ppm
Ammonia is one of the transition constituents generally present in the water before it is converted through biochemical reaction by the bacteria to turn it into nitrite. It is present in areas whereby there is high generation of waste and since it is extremely soluble in water, it can react to form ammonium hydroxide. Ammonia also comes from the agriculture sector whereby uncontrolled use of fertilizers is causing the chemical to leach into water bodies and streams. Ammonia in the form of urea is often added in order to promote the growth of bacteria in activated sludge systems in wastewater treatment plants. Apart from that borate, iron, potassium and fluoride are also included in this class.

Class 3 – Tertiary constituents exceeding 0.01ppm
Usually some of the materials included in this group does not occur naturally in the environment and some are toxic in nature. Arsenic is a good example and it is usually present in wastewater coming from mining activities. Removal of arsenic is quite complicated and unless it is present in organic form, which allows removal by coagulation or absorption through use of carbon, arsenic can remain in the water and pose huge danger and threat if it reaches drinking water supply. Barium is also another constituent not normally found and it can be removed through conventional lime treatment process. Copper can sometimes present in elevated level depending on which type of water supply piping that are being used. The compound is also deliberately added for the purpose of algae control but however, government regulations will limit its level in drinking supply at 1ppm limit max.

Class 4 – Trace constituents not exceeding 0.01ppm
Cadmium, Cobalt, Mercury, Cyanide and Nickel are generally grouped here. One particular cause of concern is the presence of cyanide which comes from the plating industry and its presence with over 0.2ppm level in the wastewater can pose serious health concern issues. Cyanide can be removed via activated sludge system or through use of resin in anion exchange. Mercury is generated from the battery and electroplating factory can also be removed via filtration process. These trace constituents can produce undesirable toxic poisoning effect that is harmful to the human body.

Class 5 – Transition constituents
Materials under this class can have varying concentration depending on which stage it is at during the biological activity. Common examples are like carbon cycle, nitrogen cycle, oxygen cycle and sulfur cycle.