Safe and effective operation of wastewater plants

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Jul 920 min read

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Technical Review | Open Access | Published 16th July 2024

Safe and effective operation of wastewater plants

Tim Sandle | EJPPS | 292 (2024) | Click to download pdf

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Introduction

Wastewater treatment plants are present within many pharmaceutical facilities, due to the scale of operations and to ensure that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans¹. Due to pollution concerns, and the associated impact of antimicrobials leading to an increase of multi-drug resistance², it is important industrial effluent receives at least pre-treatment (and most commonly full treatment) at the actual plant. This is in order to reduce the pollutant load, before discharge to the sewer. This process is generally referred to as industrial wastewater treatment. This is necessary because industrial wastewater, as with many pharmaceutical facilities, will invariably contain pollutants which cannot be removed by conventional sewage treatment. It also stands that the variable flow of industrial waste associated with production cycles may upset the population dynamics of biological treatment units, such as the activated sludge process.

The function of such plants is to clean sewage and water so that they can be returned to the environment, without polluting other water sources or supplies. Wastewater plants are designed to remove solids and pollutants, break down organic matter and restore the oxygen content of treated water. Ideally, with wastewater treatment plants a pharmaceutical compound and its metabolites undergo a partial or complete mineralization or a slow biodegradation after binding on solid sludge. In poorly performing plants, pharmaceuticals pass unchanged through the wastewater treatment plant and into the general water system or into the environment³.

Key aspects of the treatment steps, together with some examples of operational problems, are discussed in this paper, together with some examples of what happens when things go wrong. A companion paper to be published in the next issue of this Journal considers microbial control aspects of wastewater treatment,

Wastewater treatment and pharmaceuticals

Wastewater segregation and treatment at the source are favoured for pharmaceutical facilities. This is because of the need to eliminate persistent micropollutants before they enter the general water supply, and to direct the costs of doing so to the pharmaceutical facility rather than requiring a water municipal company to undertake centralized end-of-pipe treatment⁴.

While many pharmaceutical companies take strides to ensure that they do not add pollutants into water supplies, unfortunately active compounds from pharmaceutical production remain widespread pollutants in the aquatic environmental⁵. To assess the risk, pharmaceutical companies should undertake an evaluation of compound degradability, in terms of half-lives, in order to develop appropriate integrated solutions for mitigation of pollutants entry into the water cycle. An example of addressing pollutants is through the use of higher hydraulic retention times, although there are other measures.

Operation of wastewater plants

There are three main stages of the wastewater treatment process, aptly known as primary, secondary and tertiary water treatment. In some applications, more advanced treatment is required, known as quaternary water treatment.

Preliminary treatment

Before the main stages, preliminary treatment is often undertaken. The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of subsequent treatment units. Preliminary treatment operations typically include coarse screening, grit removal and, in some cases, comminution of large objects. In grit chambers, the velocity of the water through the chamber is maintained sufficiently high, or air is used, so as to prevent the settling of most organic solids. Grit removal is not included as a preliminary treatment step in most small wastewater treatment plants. Comminutors are sometimes adopted to supplement coarse screening and serve to reduce the size of large particles so that they will be removed in the form of a sludge in subsequent treatment processes. Flow measurement devices, often standing-wave flumes, are always included at the preliminary treatment stage.

Primary wastewater treatment

The objective of primary treatment is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation, but colloidal and dissolved constituents are not affected.

During primary treatment, wastewater is temporarily held in a primary settling tank (or primary sedimentation tank) where heavier solids sink to the bottom while lighter solids float to the surface. Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours.

After settling, the materials are retained while the remaining liquid is discharged into the more rigorous secondary phase of wastewater treatment. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units. In more modern plants, to assist the process of retaining the heavier solids, tanks are equipped with mechanical scrapers which function to drive collected sludge (sometimes called slurry) in the base of the tank to a hopper which pumps it to sludge treatment facilities. This process of physical wastewater treatment includes sedimentation, enabling the suspension of insoluble or heavy particles from the wastewater. With this, solids such as stones, grit and sand are removed from wastewater by gravity. This occurs when density differences are sufficient to overcome dispersion by turbulence⁶.

Once the insoluble material settles down at the bottom, the plant can be run to separate the pure water. Sometimes the sludge requires treatment in order for it to be safely disposed of. This includes dewatering of sludge from industrial wastewater or the sewage plant where the residual moisture in dewatered solids determines the disposal costs and the concentrate quality determines the pollution load returned back to the treatment facility. Control of sludge is important since many drugs may be removed from wastewater by adsorption onto solids, the potential remains for compounds to then enter the aquatic environment, in particular groundwaters, via sludge application to land, landfilling, or soil erosion⁷.

In large sewage treatment plants, primary sludge is most commonly processed biologically by anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize the organic material in sludge, thereby reducing the volume requiring ultimate disposal, making the sludge stable (non-putrescible) and improving its dewatering characteristics. Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 meters deep. The residence time in a digester may vary from a minimum of about 10 days for high-rate digesters (well-mixed and heated) to 60 days or more in standard-rate digesters. Gas containing about 60 to 65% methane is produced during digestion and can be recovered as an energy source. In small sewage treatment plants, sludge is processed in a variety of ways including aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds, in-process storage (as in stabilization ponds), and land application.

Secondary wastewater treatment

The secondary treatment phase of wastewater is designed to substantially degrade the biological content of the waste through aerobic biological processes. The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter.

There are different ways to achieve this, depending upon the plant design, such as:

1. Biofiltration. The process of biofiltration uses sand filters, contact filters or trickling filters to ensure that any additional sediment is removed from the wastewater. A biofilter is a bed of media on which microorganisms attach and grow to form a biological layer (a biofilm)⁸. Biofiltration is thus usually referred to as a fixed–film process. Generally, the biofilm is formed by a community of different microorganisms (bacteria, fungi, yeast, etc.), macro-organisms (protozoa, worms, insect's larvae, etc.) and extracellular polymeric substances (EPS). Biofiltration processes are usually aerobic, and microbial action is a key-factor of the process performance⁹.

The main drawback with biofiltration is the tendency for the accumulation of biomass in the filtering media, leading to bioclogging and flow channelling.

2. Aeration: This is a lengthy process which increases oxygen saturation by introducing air to wastewater, normally through air circulation to increase the oxygen content in the water. Typically, the aeration process can last for up to 30 hours.

3. Oxidation ponds: This process is generally reserved for warmer climates. The method utilizes natural bodies of water such as lagoons, enabling wastewater to pass through for a set period before being retained for two to three weeks.

An aerated lagoon (sometimes called an aerated pond) is a wastewater treatment system consisting of a pond with artificial aeration to promote the biological oxidation of wastewaters. Lagoons can be aerated through one of the following mechanisms¹⁰:

Motor-driven submerged or floating jet aerators,
Motor-driven floating surface aerators,
Motor-driven fixed-in-place surface aerators,
Injection of compressed air through submerged diffusers.

These different types of aerators transfer air into the basins required by the biological oxidation reactions, and they also provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microorganisms). Given that all biological oxidation processes are sensitive to temperature most surface aerated vessels operate at between 4 °C and 32 °C ¹¹.

Alternatives to lagoons are:

Activated sludge,
Trickling filters,
Rotating biological contactors
Biofilters.

Each of the above, including the lagoon, function in the same way: they use of oxygen and microbial action to reduce the pollutants in wastewaters.

Rotating biological contactors (RBCs) are fixed-film reactors similar to biofilters in that organisms are attached to support media. In the case of the RBC, the support media are slowly rotating discs that are partially submerged in flowing wastewater in the reactor. Oxygen is supplied to the attached biofilm from the air when the film is out of the water and from the liquid when submerged, since oxygen is transferred to the wastewater by surface turbulence created by the discs' rotation. Sloughed pieces of biofilm are removed in the same manner described for biofilters. A rotating biological contactor is a biological treatment process used in the treatment of wastewater following primary treatment. The primary treatment process means protection by removal of grit and sand and coarse material through a screening process, followed by a removal process of sediment by settling¹².

4. Biological processes are used to break down the organic matter present in wastewater, such as soap, human waste, oils and food (converting organic compounds into carbon dioxide, water, and biosolids). Through this, microorganisms function to metabolize organic matter in the wastewater in biological treatment. There are three stages:

a. Aerobic processes: Bacteria decompose the organic matter and convert it into carbon dioxide that can be used by plants. Oxygen is used in this process.

b. Anaerobic processes: Here, fermentation is used for fermenting the waste at a specific temperature. c. Oxygen is not used in the anaerobic process.

Composting: A type of aerobic process where wastewater is treated by mixing it with sawdust or other carbon sources.

5. Chemical treatment involves the use of chemicals, added to the water, such as chlorine (including hypochlorite) or ozone, which is an oxidizing chemical. The purpose of adding chemicals is to kill bacteria. Following chemical treatment, neutralization is undertaken through the addition of an acid or base to bring the water to a pH of 7.

Ozone wastewater treatment requires the use of an ozone generator, which decontaminates the water as ozone bubbles percolate through the tank.

The four main types of chemicals used in wastewater treatment are:

pH neutralizers,
Anti-foaming agents,
Coagulants,
Flocculants.

With the biological and chemical processes, biodegradation tests can be performed such as the closed bottle test or the Zahn–Wellens test. In general, these tests are carried out with several hundred milligrams of a substance as the carbon source; however, they require considerable quantities of drugs to be present in order to be effective at detecting contamination¹³. In general, molecules with long, highly branched side chains are generally less amenable to biodegradation than unbranched compounds with shorter side chains¹⁴.

Following a successful secondary wastewater treatment, water is generally allowed to be released into the local environment, since the biodegradable contaminants have been reduced down to safe levels. However, sometimes further treatments are required since some dissolved nutrients such as nitrogen and phosphorous may remain following the secondary treatment stage.

Tertiary wastewater treatment

Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed. Individual treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. The intention of tertiary wastewater treatment is to increase the quality of the water to domestic and industrial standards, or where tighter requirements are required for the safe discharge of water (such as the removal of microbial pathogens). The tertiary phase is generally reserved to produce water suitable for drinking purposes.

Tertiary treatment includes the use of micro-filtration or by filtration using synthetic membranes. Following membrane filtration, the treated wastewater has nitrates removed via microbial denitrification¹⁵. Nitrification, the oxidation of ammonia via nitrite to nitrate, has always been considered as a two-step process catalysed by chemolithoautotrophic microorganisms oxidizing either ammonia or nitrite. Nitrification is catalysed by ammonia-oxidizing bacteria or archaea and nitrite-oxidizing bacteria¹⁶. One problem that can occur relates to high nitrogen levels. The biological reduction of nitrate to nitrogen gas is performed by bacteria that live in a low-oxygen environment. To thrive, the bacteria need biochemical oxygen demand (BOD) – soluble BOD, as previously discussed. Particulate BOD needs to be broken down into solution before it is of value. If the plant experiences high treated effluent nitrate levels it is usually because of one of the following reasons:

Adequate carbon source. Denitrifying bacteria requires a considerable amount of soluble BOD (some five times as much as the amount of nitrate being denitrified) and many facilities find it difficult to provide an ongoing supply of readily digestible BOD
Wastewater cannot be denitrified unless it is first nitrified. Ensure that the nitrification process is working otherwise there will be no nitrate to denitrify.
Ensure that the anoxic tank has a DO = 0.0mg/l. If this reading is higher than 0 mg/l, the Mixed Liquor Return Rate (MLR) might need to be reduced.

For pharmaceuticals, research suggests that for the tertiary treatment stage compounds are not easily eliminated by sand filtration; however, technologies like ozonation and microfiltration/reverse osmosis (MF/RO) processes employed are far more effective against pharmaceutical micro-pollutants¹⁷.

With some wastewater processes, the water undergoes additional disinfection. Disinfection normally involves the injection of a chlorine solution at the head end of a chlorine contact basin. The chlorine dosage depends upon the strength of the wastewater and other factors, but dosages of 5 to 15 mg/l are common. Ozone and ultraviolet irradiation can also be used for disinfection but these methods of disinfection are not in common use. Chlorine contact basins are usually rectangular channels, with baffles to prevent short-circuiting, designed to provide a contact time of about 30 minutes. However, to meet advanced wastewater treatment requirements, a chlorine contact time of as long as 120 minutes is sometimes required for specific irrigation uses of reclaimed wastewater. The bactericidal effects of chlorine and other disinfectants are dependent upon pH, contact time, organic content, and effluent temperature.

Specialist treatment for pharmaceuticals

Separate to more general wastewater treatment, the processing of synthetic organic materials, pharmaceuticals, detergents and so forth can be very difficult. Treatment methods are often specific to the material being treated. These additional methods include advanced oxidation processing, distillation, adsorption, ozonation, vitrification, incineration, chemical immobilisation or landfill disposal¹⁸. Given that some materials such as some detergents may be capable of biological degradation, this means a modified form of wastewater treatment can be used.

Many waste products from a pharmaceutical process include acid and alkalis. Acids and alkalis can often be neutralized under controlled conditions. However, neutralization invariably produces a precipitate that will require treatment as a solid residue and this by-product may be toxic. This can include the generation of noxious gases, and these may require additional treatment. Overall, other forms of treatment are typically required following neutralization.

With other materials, much is dependent upon the type of pharmaceutical product. Different factors affect how well a wastewater plant can deal with a pharmaceutical ingredient. With compounds showing a sorption coefficient (Kd) of below 300 L kg−1, sorption onto secondary sludge is not relevant and their transformation can consequently be assessed simply by comparing influent and effluent concentrations¹⁹. However, with higher sorption coefficients the process is more complex.

Toxic materials, like organic materials, metals, acids, alkalis, non-metallic elements are generally resistant to biological processes unless very dilute. This requires additional measures, such as addressing traces of metals by precipitating them out by changing the pH or through treatment with other chemicals. Where toxic materials are resistant to treatment or mitigation the only redress is to concentrate them and then send them for landfilling or, ideally, recycling.

In terms of the different types of wastewater treatment, as discussed above, a study conducted in the U.K. by Kasprzyk-Hordern and colleagues²⁰ found that utilizing trickling filter beds resulted in, on average, less than 70% removal of a range of discharged pharmaceutical products, whereas utilizing activated sludge treatment was far more effective, producing a far higher removal efficiency of over 85%.

Protective measures

With pharmaceuticals, parts of the water treatment process can lead to blockages or damage unless action is taken. Waste streams rich in hardness ions as from de-ionisation processes can readily lose the hardness ions in a build-up of precipitated calcium and magnesium salts. This precipitation process can lead to the furring of pipes and sometimes the blockage of disposal pipes. This risk is overcome by the periodic treatment of de-ionisation waste waters and disposal to landfill or by careful pH management of the released wastewater.

Addressing corrosion

There are numerous metal corrosion mechanisms under aqueous conditions. General, localized, microbiological, under deposit, and flow assisted corrosion are several common examples. The specific type of corrosion depends on the local environment and water composition. There are two general reactions that occur at the metal interface during a corrosion event. The anodic reaction is where the electrons within the metal are trying to find an active site to transfer their electrons to the cathodic reaction. This is partly driven by a metal’s thermodynamic drive to achieve the highest oxidation state possible. At the cathodic reaction, oxygen or an oxidizer (such as bleach) are willing to accept the metal’s electrons converting oxygen or oxidizer into a hydroxide ion for oxygen, chloride for bleach, or other anions for alternative oxidizers. The salts in the aqueous medium act as a bridge for the electron reaction and help drive the reaction by facilitating the dissolution of metal cations. Chlorides and sulphates are examples of anions that accelerate corrosion reactions due to their ability to form soluble salts with metals. When the anodic and cathodic reactions occur equally on a metal surface this results in general corrosion. The reactions can become localized to a small area on the metal surface resulting in localized corrosion or pitting. Microorganisms often attach to metals surfaces forming biofilms and cause localized corrosion as their respiration process generates chlorides and sulphates. When a deposit forms on a metal surface, it results in a high concentration of anions at just beneath the edge and accelerates the corrosion reactions²¹.

Corrosion is a problem that is often tolerated or mitigated in industrial systems. If a corrosion control programme results in general corrosion, the life expectancy of the metal can be estimated and appropriately planned for replacement. The most detrimental form of corrosion is localized or pitting. Here, the corrosion process occurs at an accelerated rate in a small area. In a short and unanticipated time, localized corrosion can form sizable holes in metal piping. Metal failure can occur resulting in the potential for unsafe leaks, pipe bursting, or cross contamination within an industrial process. Corrosion reaction products can also form scale. Scale in industrial systems can result in reduced flow within a pipe or interfere with industrial processes that require heat transfer in a heat exchanger. Scale can also promote microbiological growth that can form a biofilm. Similarly, biofilm can reduce flow and interfere with the heat transfer process. Scale and microbiological growth each promote localized corrosion reactions.

The approach to mitigating corrosion changes regionally since water quality changes as do environmental permits. Water ion composition varies significantly depending on the geographical location, geological environment, and water source. The more chlorides and sulphate present in the water, the more challenging the water is to mitigate corrosion. Similarly, more hardness (calcium and magnesium ions) can make the water easier to handle if there is enough carbonate or M-alkalinity in the water. Corrosion control programmes are tailored specifically based on the water quality and system. Depending on local permits, one may be able to use a metal like zinc or phosphate or phosphorus-containing chemistries for corrosion control. Zinc and phosphorus restrictions may exist in one region versus another and an alternative approach would need to be adopted for a particular water stream²¹.

In terms of the conventional solutions to address corrosion, for industrial systems, this was generally the use of heavy metals, such as chromate or molybdate. These were very effective inhibitors but were eliminated from industrial use due to the exposure of their detrimental side effects to the environment and people. The replacement of the use of heavy metals led to the development of anionically charged organic polymers and phosphonate chemistries. Through injection with these treatments, one could use inorganic phosphate as a corrosion inhibitor. With the discovery of sulfonic acid polymers, phosphate-based programmes became very effective for industrial systems. The phosphate would interact with the metal surface and form insoluble metal phosphate or the calcium in the water would form passivating film on the surface. Phosphate programmes became very effective and were considered environmentally friendly and benign to people. The negative aspects of the use of phosphate is that it is a micronutrient that can promote algae blooms or microbiological growth. Additionally, even with advanced polymer technology, conditions exist where scale and fouling from a phosphate programme cannot be mitigated effectively.

When industrial systems made the switch away from chromate programmes in the 1970s and 80s, it was a significant advancement. The progression away from phosphate and zinc is a similar achievement for industrial systems. An alternative is an E.C.O.Film (Engineered Carboxylate Oxide) technology, which is a non-phosphorus corrosion control programme. E.C.O.Film technology does not use phosphate as a corrosion control agent and lowers the amount of phosphorus in the effluent. This reduces the scaling tendency within the system and helps mitigate algal blooms by limiting micronutrients²².

Sampling and testing

The sampling and analytical procedures used are important for determining how well a waste-water treatment facility is working. Influencing factors in relation to the robustness of test results include: period of the year (month or season, year), sampling type (grab, time or flow proportional), nature of the sample (raw sewage, pre-treatment effluent, primary, secondary or tertiary effluents), water fraction analysed (dissolved, particular, raw or total), description of the analytical method (extraction and purification steps, chromatographic analysis, use of internal standards), and description of the performances of the analytical method (recovery, relative standard deviation, limits of detection and quantification)²³.

Why wastewater plants sometimes go wrong

There are several reasons why wastewater plants sometimes go wrong. These include:

1. Variation in turbidity: When plants begin to experience a variation in turbidity - the cloudiness of water due to the presence of a large number of particles - it can have negative effects on the quality of the process and effluent from the plant. When the turbidity is too high for the plant to remove it efficiently, it often carries over to production and can be present in the discharge, contaminating the process.

2. Variation in flow: If an industrial facility is not equipped to handle flow variations, this can lead to upsets to the system that will carry turbidity over and plug any downstream filters. Hence, understanding what peak demand is and using holding tanks to try to buffer out the peak demands is one way to prepare for flow variations.

3. Changing feed chemistry: Many types of waters have seasonal variations in water chemistry. Industrial plants need to be very careful in the design of any raw water treatment systems to be large enough to handle these changes, such as with the levels of iron or silica changing seasonally. It is important for plant managers to understand the variations of the contaminants feed water chemistry and design a system accordingly. Physical chemical processes to remove the iron and silica typically involve an oxidation chemical (such as oxygen) and an aluminium- based coagulant such as alum that will precipitate out the iron and silica and allow them to settle so they can be removed in a clarification filtration system.

4. Secondary waste: One of the biggest mistakes made in designing raw water treatment plants is not looking carefully at the secondary waste generated by the process. Contaminants from the feed water impact the volume and processing requirements in secondary waste. Also, sometimes these secondary wastes need to be treated and discharged, yet many times they are discharged to a publicly owned treatment works or wastewater facility and they must meet the requirements of that facility.

5. High levels of ammonia: Where high ammonia levels are measures, it is important to ensure that the plant has the following in place:

a. Generally, nitrification occurs only under aerobic conditions at dissolved oxygen levels of more than 1.0 mg/L

b. Nitrification requires a long retention time.

c. A low food to microorganism ratio (F:M)

d. A high mean cell residence time (measured as MCRT or Sludge Age)

e. Adequate pH buffering (alkalinity)

6. Failure to meet phosphorus targets: In order to reduce total phosphorus there are a number of chemical dosing options that can be pursued. It is essential at the outset to evaluate pre- and post- precipitation, and jar testing provides a quick analysis to help determine the most effective chemical needed for the process (e.g., alum or ferric chloride). Further:

a. Pre-precipitation. When identifying a chemical dosing point before the biological process, phosphorus is removed in the primary settling tanks. In conjunction, the online phosphate measurement should be taken between the primary settling and aeration tanks to be used as part of a system based on feedback control.

b. Post-precipitation. When chemical dosing is done after the biological process, phosphorus is removed in the final clarifiers or effluent filters. The online phosphate measurement should be taken between the aeration tanks and the final clarifier, or after the final clarifiers with feedback control.

c. Simultaneous (pre- and post-precipitation). This option utilizes chemical dosing before and after the biological process, which facilitates low effluent phosphorus limits.

Summary

This paper has outlined the importance of wastewater treatment, considering general operations and looking at the subject from the pharmaceutical perspective, where there are added reasons for ensuring that water is ‘clean’ before it is released into the environment (not least due to the pollutant capacity of some active ingredients and the role of some compounds in promoting antimicrobial resistance).

A wastewater treatment plant cleans water so that it can be returned to the environment. These plants remove solids and pollutants, break down organic matter and restore the oxygen content of treated water. They achieve these results through four sets of operations: preliminary, primary, secondary and sludge treatments, as set out above.

Sometimes plants do not operate as designed and the paper has outlined some of the physical and chemical reasons for this.

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