Sewage is made up of carbonaceous waste and a small amount of nitrogen waste. Nitrogen is a found in proteins and in ammonia, an important compound in sewage. In domestic sewage the ammonia could make up about a half of the total BOD. Urine is the major source of ammonia in domestic sewage. Ammonia needs a lot of oxygen and certain bacteria to decompose it to nitrate. This process is called nitrification.

There are a large number of bacteria that carry out carbonaceous oxidation but nitrifying bacteria require special attention. This is because the numbers of nitrifying bacteria are low, reproduction is slow, oxygen requirements are high and reaction with ammonia is slow and reduced further in cold weather.

Nitrate is a compound containing nitrogen and oxygen. Complete nitrification occurs when all ammonia has been converted to nitrate. The nitrate is then broken down via a process called denitrification. Denitrifcation occurs under anoxic conditions, when there is no dissolved oxygen in the sewage, and the bacteria utilise the oxygen in the nitrate and release nitrogen gas, as bubbles. If these bubbles attach themselves to sludge it may be lifted and float to the surface.

Denitrification can be a nuisance if it occurs in the wrong place. Anoxic zones or tanks are sometimes created to ensure denitrification occurs where it can be properly controlled. Anoxic conditions can occur in final settling tanks. This can cause rising sludge when the liberated nitrogen gas buoys up the sludge and lifts it to the surface resulting in high SS in the effluent. Nitrate compounds behave as nutrients when discharged to rivers and can encourage the growth of algae. The EA sometimes requires nitrate to be removed from sewage effluent and the consent may place limits on nitrate compounds. When this is the case, nitrification and denitrification will form part of the treatment process.

Bulking can be said to have occurred in activated sludge plants when the sludge does not settle easily and has an excessive volume. This can lead to carry over from the final effluent clarifies. A bulking sludge is usually characterised by a sedimentation rate of less than 0.3 m/h, an SSVI or SVI of above 120 and 180 ml/g respectively and a low density structure.
The cause of bulking is normally due to either excessive filamentous growth or micro-organisms producing extracellular such as polysaccharide.
There are a number of short-term control measures which include biocide addition, use of flocculating chemicals or increasing the RAS flow. These measures generally treat the symptoms and not the underlying cause of the problem. The production of extracellular material and excessive filamentous micro-organisms have been related to nutrient deficiency, low dissolved oxygen and configuration of the aeration basin. This article deals with the control of activated sludge bulking based on these causes.

Nutrient Deficiency
A bulking sludge with a nutrient deficiency can be identified via a simple wastewater analysis of the influent wastewater and comparison of the BOD, N and P concentrations. Ideally the ratio of BOD:N:P should be 100:5:1. The limiting nutrients are generally N and P. This ratio however is the maximum requirement of most plant and many plants operate successfully at lower ratios. A bulking sludge with a nutrient deficiency is often viscous due to excessive production of extracellular polysaccharide and may also contain filamentous types 021N and Thiothrix.

Factors within the aeration tank can effect the concentration and requirement of nutrient dosing. Firstly, a decrease in temperature results in more nutrients being needed for carboneous removal as the BOD load is used for cell maintenance. Secondly, high sludge age results in lower nutrient requirements due to cell lysis. Thirdly, nutrient dosing may be required in instances were the BOD is readily available (eg it is present as a simple sugar) but the N and P are organically bound and are therefore not available for utilisation at a sufficiently high rate. Fourthly, when treating a variable carbonaceous loads with a tendency to nutrient deficiency the nutrient should be dosed continually to reduce the impact of shock loads. Yet it is possible to overdose N if only the concentration of ammonia is monitored in the effluent and nitrification occurs.

In general a soluble inorganic N and P concentration of 0.5 to 1 mg/l should be maintained within the aeration tank. However where a highly soluble BOD load is being treated the N and P minimum concentration may have to be increased to 1 to 3 mg/l. Some plants use commercial fertiliser which contains urea, ammonia and nitrate. However in these mixes the nitrate can lead to denitrification, therefore formulas without nitrate should be used.

Low Dissolved Oxygen Concentrations
A high F:M ratio requires a high DO concentration for effective treatment of the BOD load. However at low DO concentrations and at high F:M ratios excessive filamentous growth may occur leading to bulking. The filamentous micro-organism associated with low DO are S. natans, type 1701, M. parvicellaand possibly H. hydrossis.
Bulking due to low DO can occur in a short space of time. A correction mechanism could be to increase the DO in the aeration tank. However this could lead to nitrification due to excess aeration and also for washout of the filamentous micro-organisms three sludge ages are required. Alternatively the F:M ratio could be decreased which would mean an increase in MLSS concentration. An increase in the MLSS concentration may lead to nitrification, due to increased sludge ages, or may exceed the secondary clarifiers design load. There are only really two solutions to this problem to either use a biocide or modify the aeration basin.

Aeration Basin Design
Activated sludge plants, which operate under complete mix system, produce poor settling sludge than systems which incorporate a selector box. In such systems a high substrate concentration zone is artificially created by mixing the RAS with the influent wastewater. This favours the growth of the floc-forming bacteria over filamentous micro-organisms. Floc-forming micro-organisms are favoured due to their high bioadsorption ability which limits the substrate available for other micro-organisms. However if the bioadsorption ability of the RAS has not been regenerated before contact with the influent then the efficiency of this process will be reduced.
Selector boxes can be either anoxic, aerobic or anaerobic. Anoxic zones are primarily used at nitrifying plants to remove nitrate and are effective due to floc-forming micro-organisms ability to denitrify and bioadsorb at high rates. An aerobic selector box should be based on a design F:M of 12 kg COD/kg MLSS, while anoxic selector boxes design load should be 1.2 kg COD/kg MLSS both with a retention time of 20 minutes. Anaerobic selector boxes have a similar F:M load rate to anoxic selector boxes but the retention time is between 45 minutes and two hours.
The above strategies provide starting points for activated sludge bulking control. However to understand the cause of bulking on an activated sludge plant the causative micro-organisms and the operating conditions which give rise to bulking should be first identified.

A question often posed by operators of anaerobic digesters is “what should I be looking for?”. This is often in the context of process plant with a myriad of expensive instrumentation and computer control. I have always been of the view that anaerobic digestion is a simple process looked after by nature and with its slow reaction rate needs little attention, but in the words of the management guru: “measure more by measuring less”.

Ratio 1 – Gas : Sludge
There are 2 ways of measuring gas production : the instrumentation way and the operator way. If you can rely on your gas meter, please use it, but there is another way. Nowadays, for safety reasons and to reduce greenhouse gas emissions, all biogas is burned, either in boilers, gas engines or waste gas burners. The trick is to calibrate the consumption rate of each gas appliance (m3/hr) and to ensure that there is an hours run meter for each appliance. There is no need to monitor gas consumption more regularly than daily, so the daily gas production from the digester(s) is the sum total of the hours run by each appliance and its gas consumption rate. A log book and a calculator or a spreadsheet will do the job for you.
Likewise, there are 2 ways of measuring the raw sludge feed to the digester : the instrumentation way and the operator way. If you are unhappy with reliance on a series of flowmeters, calibrate your feed pump(s), record the hours run, and there you are with the daily raw sludge feed volume.
Now you can calculate Ratio 1 – divide the daily (or weekly or whatever time interval you choose) gas production by the raw sludge feed volume. What is the significance ? If your raw sludge is consistent, then the ratio will be. If the ratio is not consistent day by day, either your raw sludge dry matter is varying or your digester is not performing, because Ratio 1 is approximately equal to:

(Raw Sludge % TS) x (% VS) x (% Volatile Destruction) / 1000

For example a raw sludge with a TS of 6%, a VS of 80% and a VSD of 45% will give a value for Ratio 1 of 21.6.

Ratio 2 – Gas : Digester Capacity
Again monitor the gas production, expressed as m3/day. Divide this figure by the digester capacity (m3) to give you your second ratio. This tells you how hard your digester is working.
In the example above a digester working on a 15 day retention time will give a value for Ratio 2 of 1.44. A figure less than 1.0 means that you could be working your digester much harder; a figure more than 1.5 is good for a conventional sewage sludge digester; high solids digesters have been known to achieve a figure of more than 4.

Ratio 3 – CH4: CO2
There are two ways of monitoring gas quality – yes, you guessed it, using an expensive instrument, either permanent or portable, to measure the CH4 content of the gas, or a “Draeger” tube with a portable set of bellows to measure the CO2 content; it can be assumed for this ratio that the biogas comprises only CH4 and CO2. Whether you measure CH4 or CO2 the important issue is that Ratio 3 is consistent from day to day; for a sludge digester it is normally about 1.5. The alarm bells should ring if Ratio 3 falls because this indicates a rising CO2 content, which in turn indicates that the methane forming stage of the anaerobic digestion process is becoming less stable, which might lead to failure. Measuring the pH will tell you that your digester has already failed and measuring VFA’s is more complex.

Measure the 3 ratios regularly; if your digester is happy the ratios will not vary.
Anaerobic Digester
Diagram of anaerobic digester

Michael Chesshire, Greenfinch Ltd, Burford House, Tenbury Wells, Worcs, WR15 8HQ (Tel: 01584 810777)

One of the most fundamental control parameters for the activated sludge process is the relationship between the load (i.e. kg/day as opposed to mg/l) of BOD (or bacterial ‘food’) entering the aeration plant, and the ‘mass’ of bacteria in the aeration tank available to treat the incoming BOD. This is therefore known as the Food to Mass ratio (F:M ratio), also often referred to as the Sludge Loading Rate (SLR).

The amount of biomass within the reactor is known as the Mixed Liquor Suspended Solids (MLSS). Having established the MLSS by filtration and drying at 105 °C to constant weight, it is then possible to calculate the F:M ratio using the following simple model.
FM
FB:M = [(Bi x Qi)/SMLx Va)] x 10-3 x 24
Where:

Influent flow = Qi (m3/hr)

Influent BOD = Bi (mg/1)

Aeration tank volume = Va (m3)

MLSS = SML(g/l)

In this instance, FB:M refers to the loading measured as BOD5as opposed to FC:M which is when COD is the measured parameter. This very important ratio is perhaps the single most important parameter in controlling the activated sludge process, and is defined as the kg of BOD5applied per kg MLSS per day.
Equation
The ABC Paper Co Ltd operates an activated sludge plant for the treatment of their tissue mill waste effluent. Discharge is to an estuary which requires high BOD removal but there is no ammonia element to the consent and hence no requirement for nitrification. The plant operates with the following parameters: Flow rate: 6000 m3/day (250 m3/hour), Influent BOD: 300 mg/l, MLSS: 3000 mg/l (3.0 g/l) and Aeration tank volume: 3000 m3.
Simon J Binyon, ARL Consulting Ltd, Tel: 01829-741524.

Environmental concerns about the release of greenhouse gases to the atmosphere, and issues of global warming and climatic change regularly feature in the media. What is often not referred to in these stories is the role of wastewater treatment. Biological processes could have a major effect on the composition of the atmosphere. Recent studies reveal that detectable quantities of the greenhouse gas dinitrogen oxide, N2O, is produced during both nitrification and denitrification of wastewater. For example, it is estimated thatca. 25 % of the atmospheric dinitrogen oxide in the Netherlands originates from polluted waters and sub-optimal wastewater plants. The greenhouse gas N2O has a long lifetime in the atmosphere and its greenhouse effect is much greater than CO2. Furthermore, it is the dominant source of stratospheric nitric oxide, NO, and therefore could have a significant influence on our climate.

Nitrification and denitrification are the two main steps of recycling of ammonia to dinitrogen (N2) in the nitrogen cycle. Nitrification is traditionally defined as the aerobic oxidation of ammonia to nitrate, NO3, via nitrite, NO2. This is carried out by two groups of autotrophic bacteria (bacteria that require no organic source of carbon); ammonia oxidisers (NH3aNO2), typified by the genusNitrosomonas and nitrite oxidisers (NO2aNO3), typified by the genusNitrobacter. The carbon is provided by carbon dioxide or bicarbonate. Denitrification, however, is an anoxic process carried out by a consortium of heterotroph bacteria able to use N-oxides as an alternative electron acceptor to oxygen.

Conventional systems for the treatment of nitrogen containing wastewater are traditionally based on the principal outlined above, where nitrification is an aerobic process and denitrification is restricted to anoxic conditions. These conditions are normally obtained, either by separation of the nitrifying and denitrifying processes, or, by temporal separation of each step-achieved by switching between aeration and no aeration in the same unit.

Recent studies illustrate that both pure cultures of nitrifying bacteria and nitrifying activated sludge are capable of simultaneous nitrification and denitrification. The principal mechanisms of N2O production by nitrifiers is NO2-reduction to N2O by the ammonia oxidisers, a process called aerobic denitrification. For example, the production of both the greenhouse gases N2O and NO and of N2, intermediate products and products of denitrification, are well documented gases, produced by the autotrophic ammonium oxidiserNitrosomonas. In addition, micro-organisms capable of heterotroph nitrification have been found capable of aerobic denitrification under dissolved oxygen concentrations as high as 7 mg l-1. Furthermore, the most studied aerobic denitrifier Thiosphaera pantotropha, a heterotroph nitrifier (one that requires organic carbon), exhibits aerobic denitrification rates equivalent to approximately 50 % of the rate under anoxic conditions. These findings indicate the existence of micro-organisms capable of complete conversion of ammoniacal pollution to nitrogen in a one step process. In addition, it might explain the nitrogen losses reported from mass balances on nitrifying wastewater treatment plants, and the reported release of the greenhouse gas N2O from nitrification and denitrification process plants.

Laboratory experiments have shown that up to 8 % of the nitrogen can be lost as N2O and NO during nitrification under sub-optimal oxygen concentrations (i.e. under oxygen stress). While aerobic denitrification at relatively high dissolved oxygen concentrations has been reported, most simultaneous nitrification and denitrification occurs mainly at sub-optimal oxygen levels. The ability to undertake aerobic denitrification is believed to give the ammonia oxidisers an advantage over the nitrite oxidisers. Firstly, oxygen will be conserved for the initial ammonium oxidation step. Secondly, a toxic product is removed. Finally, the competition for oxygen is decreased by the removal of the substrate for the nitrite oxidisers.

Despite environmental concerns regarding the release of greenhouse gases to the atmosphere, the development of nitrification/ denitrification processes encouraging aerobic denitrification could be advantageous for the water industry. The main benefit is the decrease in operating costs – lower oxygen requirements for the plant equate to lower energy demands. Furthermore, denitrification will be cheaper as a number of reduction steps are eliminated from the process, enabling more compact process plants to be constructed. Result – cost savings all round.

However, it must be remembered that by running the nitrification process under sub-optimal oxygen conditions, a significant amount of dinitrogen oxide will be produced. This off-gas will require secondary treatment in order to prevent its release to the atmosphere. One option could be to operate the simultaneous nitrification/denitrification step in an enclosed reactor, before passing the resultant gases through the anoxic denitrification step. Such a process could conceivably be combined with off-gas treatment for odour removal – an increasingly important consideration. This is one of the next steps forward in R&D terms.

Dr. Bettina Colliver, School of Water Sciences, Cranfield University.

At all rates of flow to the treatment works rotary distributors must be fed at rates sufficient to ensure that the jets of sewage will rotate the arms. If the flow to the distributors is too low they might stand still and dribble. If this happens sewage will pass through one spot on the filter and effluent quality may deteriorate. Rotation of rotary distributors at all rates of flow may be achieved by means of a dosing syphon. This is used to hold back the flow until there is adequate volume and head to ensure efficient and continuous operation of the distributor.
The device shown in the diagrams below consists of a dosing tank fitted with a syphon which, when primed by the rising level of sewage, discharges to the filter under sufficient head to rotate the distributor. When the flow into the chamber is less than the discharge capacity of the syphon, the water level falls in the chamber until eventually the syphonic action is broken. The chamber refills with sewage until the syphon is re-primed and this charges to the filter once more. This cyclical action is then continued until the flow through the works increases to match the capacity of the syphon.
Dosing syphons – how they work
This diagram shows adosing siphon after and before discharge. The main operations comprise:-
The dosing chamber fills, causing the air pressure in the dome to increase and sewage to be displaced from the U-tube.Sewage then siphons over into the siphon pipe and the surge fills up the U-tube again.
The level in the dosing chamber falls to unseal the air intake pipe, allowing air to enter the dome and break the siphon. The height of the U-tube, labelled B, controls the difference in sewage levels between the chamber and the dome, labelled A. If it is too short, the sewage will not rise high enough to cover the dome completely, a pocket of air will remain in the dome and the siphon will function erratically If the arm is too long, the sewage
will rise above either the siphon pipe or the overflow pipe outside the dome and dribble over to the distributor.
Construction and Maintenance
The main castings for each syphon are in cast iron, but the air pipes are usually manufactured from galvanised
mild steel. These are the weak link in the equipment as they can corrode and become blocked by grease and
fat. Inspection of dosing syphons should cover blockages in the air intake and U-tube and corrosion that might affect the lengths of these pipes. Wear, corrosion or damage to the lower edge of the dome may affect the breaking of the syphon. While there is no power or energy requirements for syphons it is essential that the air pipes are kept clean and airtight.
Failure of the dosing syphon is a common cause of noncompliance.
Filter bed performance is best if dosing occurs 8 to 12 times an hour. If possible syphons should be adjusted to achieve this dosing frequency by reducing the volume of the chamber ( bricks/concrete blocks). On small works it is sometimes difficult to achieve this dosing frequency; the frequency may be as low as once or twice per hour. If compliance is at risk recirculation should be considered.

Reed beds are a tertiary treatment with the process aim of removal of suspended and dissolved matter beyond that which the conventional secondary sewage treatment process provide. They remove 60 – 80% solids from the secondary effluent. In the operation of horizontal flow reed beds secondary treated effluent is passed uniformly over vegetation so that suspended solid matter is retained in the vegetation. Reed beds have are very good at removing BOD, ammonia and nutrients, therefore this is one of the few forms of tertiary treatment that can be used to improve poor quality secondary effluent. Very simply, it consists of an area of reeds planted in a soil or gravel medium. It traps the solids from the effluent during its travel across the bed. Reed beds planted in a soil medium are capable of removing BOD and suspended solids up to 95% with potential removal of ammonia, nitrate and phosphate. Gravel based reed beds achieve BODs of 1-4 mg/l and suspended solids of 2-8 mg/l regardless of the quality of the influent. Ammonia and nitrate reduction is also possible.
The following things need to be considered by the designer of a reed bed:-¨ land cost and availability;

• gravel size 3-10 mm;
• typical depth of 0.6 m;
• need a uniform slope of 1:100;
• gravel medium needs liner;
• need constructed inlet and outlet to contain the bed;
• appropriate distribution system to prevent channelling
• appropriate effluent collection system.

Typical sizes of reed beds are 1m2/pe, so they are more suitable for small, rural works. It is advised to plant 4 reeds/m2 . The most popular reed used is the common reed ( Phragmites australis) although other suitable varieties exist. Typical loadings are 0.2 m3/m2/day.

The Activated Sludge (AS) process was developed as an alternative to biological filters, and is particularly useful for large populations where land is at a premium. More recent research however, has shown that the process can be operated in many different modes, making it a more flexible process than biological filtration. The Activated Sludge process is an accelerated natural biological treatment process. It is a complex mix of microbiology and biochemistry involving many different sorts of bugs. In the Activated Sludge Plant (ASP) bacteria secrete sticky substances that coat the minute particles carried in sewage. The particles stick together to form flocs of gel-like material, creating a support on, and in which, the bugs exist. This is the chocolate-brown coloured activated sludge. The activated sludge is aerated to dissolve oxygen which allows the organic matter (BOD) to be utilised by the bugs. The organic matter, or food, sticks to the activated sludge. The oxygen dissolved in the water allows the bugs to use the food (BOD) and also to change the ammonia to nitrate. The tank should be big enough to allow sufficient contact time (retention time) between the sewage and the activated sludge for all the chemical changes to take place.

Return Activated Sludge (RAS)
When the Activated Sludge reaches the end of the process it is still a highly active biomass but is now mixed with purified effluent. It is transferred to Final Settlement Tanks (FSTs) to allow separation from the purified effluent which may be discharged to the river or to some form of tertiary treatment. The settled biomass, called Return Activated Sludge (RAS), is then returned to the beginning of the aeration process where it will absorb fresh sewage to start the process again. This enables the process to operate as a continuous cycle.

Surplus Activated Sludge (SAS)
As the RAS mixing with the fresh sewage will produce a gradual growth in the activated sludge present it is necessary to waste a certain quantity each day. This Surplus Activated Sludge (SAS) is wasted by continuously withdrawing some of the RAS for sludge disposal.

The rate at which sewage can be treated on a biological filter will depend on the nature of the sewage and the required effluent quality. Loading rates are best measured in terms of organic loading, kg of BOD per cubic metre per day, often abbreviated to kg/m3/d. The loading rate on conventional biological filters ranges from 0.3 – 1.5 kg/m3/d. Winter/summer performance differences in biological filters can be considerable. A 25% reduction in nitrification and a 10% reduction in BOD removal occurs when the temperature drops by 15ºC as it does from summer to winter. For this reason works are designed to comply under winter conditions. The loading rate also influences the production of sludge in the humus tanks, with a high loading rate increasing the yield of sludge. The calculation of the loading rate is very simple and is based on filter volume, influent BOD and flow rate.

Example:A biological filter treats a waste from a small rural community after primary sedimentation. The works has recently being producing poor quality final effluent. Is the filter being overloaded? Area of filter = 20 m2 Depth of filter = 2 m Daily flow to filter = 10 m3 Influent BOD = 20 kg/m3 OLR = 5 kg BOD/m3/d

This works appears to be overloaded from the above calculation.