Laboratory Control of Biological Waste Treatment

Introduction

Design and operation of effluent  treatment plants of sugar industries in India is generally based on conventional treatment units with manual operation and control. Due to inadequate design of units, untrained operating personnel and seasonal nature of industry, the performance of such plants is often far from satisfactory. 

Sugar Factory Waste Treatment

There are various types of treatment flow sheets adopted for sugar factory waste treatment. However, primary treatment followed by some form of biological treatment is the most method used in most of sugar factories.

The main performance parameters to be monitored and controlled in primary treatment are floating solids, oil and grease and pH. If the design of screen chamber is of adequate capacity and regular cleaning is done, there is effective removal of floating solids. In case of oil and grease removal, pre-separation of oil by providing separate chamber at mill house greatly enhances the removal efficiency.

The pH of the sugar waste is around 5.0 to 5.5 and may require neutralization

by adding lime during startup and stabilization of aerobic biological system. But such neutralization is not required on regular basis. However, during cleaning period there are large variations in pH due to use of acidic and alkaline chemicals and it is advisable to provide a separate holding tank for such waste with proper neutralization facility. The concentration of organic pollutants also is quite high during cleaning period. Hence arrangement should be made to mix small quantities of such neutralized waste to main flow to avoid shock loading.

Aerobic Process

It is observed that provision of anaerobic contact tank before aerobic process greatly enhances oxygen transfer rate and degradability of organic pollutants. If the main biological process has some form of activated sludge process, the operation of such plant requires thorough knowledge of the governing parameters and is often seen to be handled by unskilled persons leading to various problems and malfunctioning of the units. As such the treated effluent quality either is unsatisfactory or shows large variations in the parameter values indicating inadequate control on the process.

Activated sludge process is a complex phenomenon and depend on hydraulic and organic loading, air supply, sludge recycle and wasting schedule and number of parameters controlling biological growth like F/M ratio, MLVSS, SRT, SVI and nutrient requirements. Though basic purpose of the process is to convert the soluble and colloidal organic material into suspended biomass and its subsequent separation, the process may yield poor results, if process parameters are not properly controlled. Low DO, Low biomass concentration, accumulation of sludge in aeration tank and high soluble BOD are some of the problems with aeration tank, whereas  Sludge Bulking, Rising sludge and low solid concentration in the underflow sludge recycle indicate problems with secondary settling tank.

Design Parameters

Design of treatment units need to be checked for assumptions made about waste characteristics, flow variations, hydraulic and organic loading rates, chemical dose and aeration requirements and treatment efficiencies. It is observed that many a time inadequacy of treatment units is the cause of plant failure.

Operational Monitoring Parameters

There are number of operating parameters such as waste characteristics at different stages which include flow, DO, BOD, SS and pH values to more complex operating parameters like HRT, SVI, Return sludge ratio, SRT, wasting of sludge, nutrients and micro flora which require understanding of phenomenon, prediction of effect of these parameters on plant performance and fast corrective action.

Dissolved Oxygen

DO levels must be controlled to promote desired biological processes. Adequate DO (above 2mg O2/L) provides aerobic conditions, allowing microbes to consume organic waste. Low DO can result in anaerobic to anoxic conditions.

pH

pH value outside 6.5 to 8.5 can adversely affect the treatment process. In aerobic process, lower pH may enhance dominance of fungi in activated sludge flocs making them fluffy with reduced settleability.

Total Suspended Solids

TSS indicates the level of particulate matter suspended in water. TSS is used to adjust the volume of activated sludge.

Organic Solids

Organic solids are measured in terms of volatile solids, BOD and COD. The volatile solids are measured as the solids which get oxidized at 600 degrees centigrade in muffle furnace in 20 minutes and are determined by subtracting fixed residue from total solids. They are somewhat lower than  COD  but are used for checking COD values. The relationship between BOD and COD depends upon the degradability of organic matter. The ratio of COD to BOD is approximately equal to1 for Glucose and Starch. Whereas it is more than 4 after biological treatment. This ratio could be used to determine the performance of biological treatment as regards carbohydrates oxidation. It may happen that BOD of a sample is zero whereas its COD is high. This may point to toxicity of waste where bacteria cannot survive during the BOD test. Dilution of waste may give correct BOD level due to reduction of toxicity. Addition of proper seed may help in this regard.

BOD and COD

BOD used to measure the efficiency of removing the biodegradable organic content of sewage, whereas COD is is the direct measure of total organic content of sewage and Includes biodegradable and non-biodegradable waste. COD/BOD ratio indicates degree of treatment. This ratio for untreated sugar waste is about 2  and increases upto 4-5 as the BOD is removed during the biological process. The BOD of treated effluent in well maintained ETP should be as low as             10 to 30 mg/l and COD in the range of 100 to 150mg/l.

Nitrogen and Phosphorus.

Nitrogen and Phosphorus are the main nutrients required for biological growth. BOD:N::P ratio for aerobic process should be 100:5:1 whereas for anaerobic process it should be 100:2.5:0.5. As such generally sugar factory waste does not need any nutrient addition for anaerobic process however provision of adequate N and P through urea and super phosphate dosing may be necessary for aerobic process   If the nutrients fall short, there is a possibility of predominance of filamentous fungi which may lead to sludge bulking in aeration tank.

HRT

HRT or hydraulic retention time is determined by ratio of tank volume and flow.It indicates the theoretical time of retention of water in the tank. Actual retention time  depends on the flow regime, short circuiting or mixing efficiency and is often less than HRT.

F/M RATIO

Food to Microorganism ratio is determined as the ratio of BOD load in Kg/day and Total biomass in kgs present in the aeration tank. This is used to control bacterial growth rate to achieve desired BOD removal efficiency.

MLSS and MLVSS

MLVSS which means mixed liquor volatile suspended solids concentration is considered as the measure of biomass concentration in the aeration tank. However, as the test is somewhat complicated, MLSS or mixed liquor suspended solids concentration is often used as  a rough measure of biomass present. The ratio of MLSS to MLVSS is about 0.8 and MLVSS can be estimated from MLSS by using this relationship. However for all practical purposes, MLSS is taken as control parameter. Its value is from 2500 to 4000 mg/l.

Settled Sludge Volume

The settled sludge volume of a biological suspension is useful in routine monitoring of biological processes. For activated sludge plant control, a 30-min settled sludge volume or the ratio of the 15-min to the 30-min settled sludge volume has been used to determine the returned-sludge flow rate and when to waste sludge. The 30-min settled sludge volume also is used to determine

Sludge Volume Index

The sludge volume index (SVI) is the volume in milliliters occupied by 1 g of a sludge after 30 min settling and is expressed as ml/gm. SVI typically is used to monitor settling characteristics of activated sludge. SVI in the range of 80-150 indicates well settleable sludge, whereas SVI of more than 200 indicates bulking of sludge. A high SVI indicates poor settling sludge, which can hamper effluent quality.

SRT

SRT or Solids Retention Time denotes the theoretical time of retention of biomass in the aeration system. In complete mix condition, effective sludge separation, it is estimated from ratio of total biomass in the system to biomass leaving the system ( as sludge wastage or effluent suspended solids). High SRT as compared to HRT can give better BOD removal efficiency. HRT and SRT are equal if the is no sludge separation and recycling as in aerated lagoon.

Control parameters

ASP                            Extended Aeration         

a) MLSS                                             2000-3000mg/l                     3000-5000 mg/l

b) F/M ratio                                        0.3-0.5                                    0.05-0.1

c) Sludge recirculation ratio           0.25-0.5                                  0.75-1.5

d) SVI                                                 100-150                                 75-100

e) DO                                                  >3 mg/l                                   >3 mg/l

f) pH                                                   6.5-8.0                                    6.5-8.0

g) SRT                                                6-8 days                                 20-30 days

h) HRT                                               6-8 hrs.                                   12-24 hrs.

Anaerobic process

Recently some factories have adopted UASB or some other type of anaerobic digesters as pretreatment of sugar factory waste. For distillery waste, due to high organic contents, direct anaerobic digestion is suitable treatment option.

Anaerobic process though easier than aerobic process needs careful and constant monitoring of pH as souring of digester may upset the process. Rate of organic loading, relationship between alkalinity and volatile acids concentration and its effect on pH, COD reduction and its relation to  biogas recovery are some of the crucial issues which need to be handled by the operator.

Hence it is necessary to understand the significance of parameters governing performance efficiency of activated sludge process.

some organic compounds.

Control Indices :

The principal indices which may be used to indicate the course of digestion are

1)    Gas production

2)    Volatile acids

3)    PH

4)    Alkalinity

5)    Odour of digester effluent

6)    Volatile solids reduction

1) Gas production :

The degree of removal of organic matter is in direct proportion to the amount of methane produced. In general, the percent of methane in digester gas is in the range of 65 to 75% this makes it possible to use total gas production rather than just methane production, as an indication of the course of digestion. If the gas production trend is downward continually, the digestion process is failing.

The practical problem with using gas production as an indicator of digestion is normal variation in production. These variations may be due to change in characteristics of influent or change in temperature. Hence gas production solely cannot be relied as a short term indicator of process failure.

2] Volatile Acids

If the volatile acids concentration in a digester is increasing an unbalanced condition is indicated. The acid forming bacteria are producing organic intermediates faster than the methane formers can convert the intermediates to methane & carbon dioxide. The total volatile acids concentration increases sharply as failure commences

3] pH

The problem of using pH as an indicator of the condition of digestion is that the volatile acids may increase considerably with the little change in pH due to the buffering effect of their alkalinity in the digester. Thus, pH is not a very responsive indicator of the digester upsets as compared to volatile acids and gas production. However, pH value declines as the process begins to fail.

4] Alkalinity

Bicarbonate alkalinity is formed in the digester by the reaction of ammonia with carbon dioxide and water to from ammonium bicarbonate. Buffering capacity of alkalinity holds the pH in the desired range of 6.8 to 7.2.

Although alkalinity is essential to balanced digestion, alkalinity determination is not a responsive indicator of future digestion problems. The reason for this is the formation of volatile acid alkalinity as the bicarbonate alkalinity is destroyed. However, alkalinity and determination can be used together as an indication of the condition of digestion. The alkalinity should always be greater than the volatile acids and expressed as calcium carbonate. When volatile acids and alkalinity are equal to each other, very little buffering capacity remains and trouble is imminent.

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