5.1. Introduction
5.2. By-products and solid waste
5.3. Treatment of wastewater
5.4. Treatment of polluted air
As mentioned before in par. 1.2, generally three different types of waste are distinguished:
(1): solid waste, which may cause problems with respect to dumping grounds;
(2): wastewater, which may decrease the quality of surface waters; and
(3): volatile compounds, which cause air pollution.
Several ways may be followed to reduce the occurrence of waste:
(1): waste prevention;
(2): development of clean processing methods; and
(3): end of pipe treatment.
The most important one: the prevention of waste production, has been discussed in the previous chapters of this study. Through a careful examination of the production processes, one may identify the source(s) of pollution. The development of new and clean processing methods is the next step. In the present report this approach will not receive attention, because of the specific technical and economic know-how involved.
The third step is the treatment of the produced waste before it will be discharged into the environment. This is generally referred to as ‘end-of-pipe’ treatment which will be discussed below. Because this treatment is usually very expensive, the amount of waste to be treated “at the end-of-pipe” should be as small as possible.
Practically all by-products may be used in one way or another. The amounts of solid waste that need to be dumped can be kept small. With the exception of cooling water which in most cases is not polluted, wastewater can usually not be re-used. There are several methods of treating wastewater that can be applied before it is discharged into the sewer system or surface water. Polluted air can be filtered before discharge.
The KTPCP-project (Kasur Tannery Pollution Control Project) may serve as an example of why it is worth paying attention to environmental problems, which in this case had been caused mainly by the 160 tanneries of Kasur, Pakistan (Wegelin et al, 1993). Owing to insufficient attention to waste disposal from these tanneries, artificial and stagnant lakes had received environmental pollutants. This in turn led to major health problems, decreased crop yields (up to 50%) and to contaminated groundwater and fish.
5.2.1. Slaughterhouses
5.2.2. Tanneries
5.2.3. Dairy Industry
At slaughterhouses usually, everything produced by or from the animal, except dressed meat, is considered as by-product. These by-products are either ‘edible’ or ‘inedible’. The variety of by-products is enormous, as can be seen in the diagram in figure 5 which shows a few of the by-products of the meat industry. Ockerman and Hansen (1988) offer an extensive introduction to the use of by-products. There is a large number of publications on the use of animal by-products (National Renderers Association, 1990; Scaria, 1988; Kreis, 1978; Davis, 1985; Skrede, 1979; Mann, 1982; Pearson and Dutson, 1988; Pearson and Dutson, 1992).
In many countries, all the waste that is unsuitable for human consumption is processed by rendering companies as animal feed, glue etc. In large slaughterhouses, screening devices through which waste water has to flow prior to being treated, remove large solids such as hair, paunch manure, pieces of viscera and meat, dirt, and other materials. Some of these solids have an economic value and are rendered so as to produce a salable product. Materials of little economic value may be dumped at a landfill, spread out on the land or treated together with the solids from biological treatment processes. It is claimed by Ockerman and Hansen (1988) that in the animal processing industry about 1% of protein is lost to the sewer and that it is of economic importance to recover as much of this protein as possible. If half of the protein were recovered this would be worth about $400 million (1987 dollars).
In developing countries, some or all of these products are dumped as solid waste without any further processing or composting, or they are washed away. This causes pollution in the form of bad smell and potential water pollution, leading to health hazards. If solid waste is dumped, the possibility of using this waste is lost. Solid waste can also be handled by using the waste as fertilizer after composting. During the process of composting considerable quantities of nitrogen are lost in the form of ammonia. According to Kumar (undated), slaughterhouse wastes are ideally suited for fermentation. An important advantage of handling animal waste by fermentation is the low loss of nitrogen if the liquid is handled properly (see Table 22). Kumar (undated) investigated a number of other important avenues (Kumar, undated). Of the produced sludge, the dark solid portion which settles at the bottom (about 10%), was found to be rich in protein, fat fibre and also vitamin B12. It was free from parasites and probably free of salmonella as well. Such part of the slurry can be utilized as feed material or manure.
The remaining part of the sludge, the liquid component, can be used as irrigation water, or for fish and algae cultures. If not used in time a major part of the nitrogen from the liquid will be lost as a result of volatilization.
|
Destination solid waste |
Utilization of nitrogen |
||
|
a: Dumped |
100% lost of nitrogen |
||
|
b: Composting -> fertilizer |
Large quantity of nitrogen lost |
||
|
c: Anaerobic treatment: |
|
||
|
|
1: solid part -> animal feed |
Small nitrogen losses |
|
|
|
2: liquid part: |
Small nitrogen losses possible if diluted quickly with
water |
|
|
|
|
a: irrigation water |
|
|
|
|
b: fish culture |
|
|
|
|
c: algae culture |
|
In western countries the larger part of solid waste out of the slaughterhouses consists of sludge from the wastewater treatment plants. In the Netherlands sludge, including the sludge of the slaughterhouses and meat processing industries, may probable no longer be used as fertilizer in the future because of Dutch environmental rules, which however are still heavily debated. One argument is that this kind of sludge is being discriminated as a fertilizer only because in the Netherlands a surplus of organic fertilizers exists.
The environmental rules may lead to (financial) problems because the sludge has to be considered as waste to be disposed of which has the consequence that after obligatory dewatering, it must be brought to a dumping-ground.
In principle there are no objections against using the sludge from slaughterhouses as fertilizer, provided it does not contain toxic compounds.
Waste, produced as meat from fleshing activities (7 - 23%), can be used as animal feed if fleshing will have been done before liming. Fleshings that contain lime components are only suitable for glue production. A bigger environmental problem is caused by wet-blue shavings and trimmings that contain 3% chromium on a dry matter basis. This material is not allowed to be used at rendering plants. A small amount is used for artificial leather but most of it is dumped at special dumping grounds. The waste in these dumping grounds must be isolated to prevent toxic compounds leaving the dump e.g. by percolation water.
An alternative for dumping might be the new process of extracting chrome from wet-blue waste (Koene and Dieleman, 1987). The recovered chromium is recycled in the tanning process and the dechromed protein material can be used as a source material for glue or as an animal feedstuff (El Boushy et al., 1991). Sludge from wastewater treatment plants of tanneries is too toxic to be usable, owing to toxic compounds like chromium. This kind of sludge has to be dumped at special dumping grounds.
Except for packing material etc. and sludge (in case of wastewater purification), dairies do not produce solid waste. The potential use as fertilizer of stabilized sludge from wastewater treatment plants of dairies has no environmental limitations, provided the sludge contains no toxic compounds.
Main wastewater problems
The problems of the wastewater from the slaughterhouses, tanneries and dairies result from the discharge of:
a: large amounts of BOD (slaughterhouses, tanneries and dairies).BOD-problems can be handled, as already mentioned, by biological wastewater treatment.b: high values of NKj (slaughterhouses).
NKj can be lowered by oxidation of organic compounds (proteins) followed by nitrification: conversion of ammonium (NH4+) into nitrate (NO3-). To reduce the eutrophication potential of the wastewater, nitrate must be removed. This can be achieved by denitrification: conversion of nitrate (NO3-) into nitrogen (N2).c: chromium (tanneries).
Chromium can be handled by precipitation reactions, these are simple processes.
There are basically two types of biological wastewater treatment systems: aerobic and anaerobic systems. In Tables 21 and 22 the characteristics and the (dis)advantages of these systems are mentioned.
|
|
Aerobic |
Anaerobic |
|
Applicability |
low strength: |
low, medium and high strength: |
|
(BOD, mg/l) |
(100 - 2000 mg/l) |
(250 - > 100.000 mg/l) |
|
BOD-removal: |
93-99% |
90% |
|
NH3-conversion: |
95% |
low |
|
NO3-removal: |
90%* |
high |
*: depends on BOD-load.
In view of the high BOD-load in the wastewater of tanneries, dairies and slaughterhouses, anaerobic systems seem to be appropriate wastewater purification systems. Simple anaerobic systems may achieve 50% of BOD-purification (table 21), while high-rate anaerobic systems may result in 90% of BOD-purification (table 22). Anaerobic systems do not remove such nutrients as ammonium-nitrogen. If liquid and slurry are used as fertilizer this does not need to pose specific problems. Nutrient removal systems should be applied only if water authorities set limits for the discharge of nutrients. As in most countries this is not the case, there are no reasons for industry to make high investment costs for tertiary treatment.
|
|
Advantage |
Disadvantage |
|
Anaerobic |
* possible production of energy |
* optimal process temperature is about 30°C |
|
Aerobic |
* low process temperature |
* energy need for aeration |
Source: Hulshoff Pol, 1993.
General:
The process that may be used for the treatment of wastewater produced by the industries mentioned in this report do not differ very much from each other. In general, these systems are applied to a large extent in developed countries. In developing countries adoption rates are much lower. Especially for these latter countries, treatment methodologies and technologies should be cheap, efficient and easy to operate. Important differences of wastewater treatment in the different industries will be mentioned.
For large dairies in many developing countries, treatment of wastewater is not even considered as an option. Because in developing countries the amount of milk processed industrially is minor, wastewater problems will mainly occur at the plant site and the surrounding surface waters. This implies that dairy wastewater problems in these countries are very local in contrast to those in developed countries. The dairy wastewater problem is larger in developed countries because all milk is processed industrially. For dairies in these countries it is very important that proper wastewater treatment system are installed.
As mentioned before, the primary action to reduce pollution by wastewater discharge, is efficient water management. According to EPA (1974) this can reduce the load of wastewater ca 5 fold. After a thorough search for ways to reduce water use and wastewater production, the inevitable produced wastewater can be treated in different ways as discussed below.
Usually wastewater produced during the day has a variable composition. For the optimal performance of most treatment system it is necessary that the load is rather constant and that the plant is fed with a rather constant wastewater flow. Wastewater is therefore collected in equalization or balance tanks.
Most treatment plants follow the following steps.
Preliminary treatment. This type of treatment includes screening, skimming and settling which can lead to the recovery of by-products, grease and fat and removal of coarse solids. For an optimal performance and to avoid overload of the screening devices, it is important that large amounts of produced solids such as (hog)hair, feathers etc. are collected during the processing itself as discussed in part 2. For developing countries, the salt laden tannery effluent from the soaking process can be collected in solar evaporations pans, possibly pretreated with coagulant, after which salt can be recovered. In case of chrome tanning effluents, the wastewater that contains chromium should not be allowed to become mixed with other types of wastewater: it must be collected separately. Depending on the quality of the composite effluents, neutralising chemicals like lime alum, ferric chloride etc. should be added for an effective precipitation of chromium and removal of suspended solids in the sedimentation process. From this material chrome can be recovered, or dumped separately.
Primary treatment. This involves separation of solids in a settling tank (primary clarifier), or by flotation. The settleable solids and up to 60% of the suspended solids corresponding to approximately 35% of the BOD, can be eliminated during the primary treatment. Subsequently the solids may be treated by anaerobic sludge digestion. This produces biogas and solids that are suitable for soil conditioning and fertilization. Primary treatment is a essential activity that needs to be undertaken for a proper application of various secondary treatment systems. In case of aerobic secondary treatment, a further function of this step is the reduction of electric energy required for aeration.
Secondary treatment. This usually consists of biological treatments by means of high rate anaerobic treatment systems, anaerobic (lagoons) suitable for high organic loads, or aerobic (lagoons) suitable for low organic loads, activated sludge, oxidation ditch or a combination. Present research is mainly focused on low energy demand and low volume treatment systems and optimum process control. Usually, a combination of high rate anaerobic treatment and aerobic activated sludge is required to meet effluent quality demands. Removal efficiencies reached with these kinds of combination are up to 98-99%. Depending on the operational conditions, removal efficiencies for slaughterhouses range from 70 to more than 99% for BOD and grease and from 80 to more than 97% for Suspended Solids (SS). The process performance depends strongly on the amounts of SS that can be removed in the primary treatment phase.
Tertiary treatment. This includes chemical-physical methods such as adsorption, stripping, coagulation, sedimentation, chlorination as well as biological methods like slow sand filtration and maturation ponds. This post-treatment serve to remove nutrients such as phosphorus, sulphide, suspended solids, remaining BOD as well as pathogens.
Another method of wastewater treatment is that of irrigation on land. Before wastewater is applied on land, toxic compounds such as chromium, salt sulphide, etc. have to be removed. Small amounts of nitrate and phosphate however may serve as fertilizers. The BOD5 value is usually not allowed to be higher than 300 mg/l.
At present, this kind of wastewater treatment is carried out mainly in developing countries. The method is cheap, rather easy to perform, does not require highly sophisticated techniques and can be apllied because of the usually low pollutional strength of the produced wastewater.
Economic considerations
The costs of wastewater treatment are a factor of major importance for the selection the appropriate treatment system. Estimates should be made of the investment costs and the expected annual costs. The investment costs are largely determined by construction costs, the costs of land and the required degree of removal of pollutants. The annual costs will depend on the price of the energy and chemicals required for the operation of the plant, the discharge fees and the capital costs on investment. A problem for the estimation of the costs of treatment plants is that prices are rapidly changing. Cost estimates should therefore be referenced to an index.
From a comparison of the costs of 6 treatment systems (stabilisation ponds; aerated ponds; high rate anaerobic treatment + ponds; high rate anaerobic treatment + trickling filters; activated sludge process; and oxidation ditch; DHV, 1993) it can be concluded that high rate anaerobic treatment + post treatment of the effluent offers a very economic and effective solution.
The relatively high initial costs are compensated for by the low costs of energy and maintenance, which results in low running costs and a limited need for land. Costs of a stabilisation pond, high-rate anaerobic treatment plant + post-treatment in a pond and an activated sludge process for the sewage treatment plant for a town of 50,000 inhabitants (producing ca 550 ton BOD and ca 135 ton N) given as a reference, are: resp. around 3,5.106, 2.106 and 2.106 USD for investment costs and running cost resp. around 400,000, 300,000 and 430,000 USD on an annual basis. In this calculation it is assumed that electricity costs are 0.10 USD per kWh, sludge disposal costs 10 USD per 1000 kg and that the price of land is 25 USD per m2. Lagoons will become more economical if land costs are below 10 to 20 USD/m2. Wastewater from slaughterhouses, tanneries, and the dairy industry are more heavily loaded with pollutants than sewage. This will have the effect that anaerobic processes are more competitive than aerobic processes owing to the much lower energy costs of anaerobic treatment.
Taiganides (1987) gives an overview of relative cost indices and ranking for various treatment systems. According to him, the selection of the treatment system has to be undertaken on the basis of economic costs, environmental considerations, and the technical complexity of the system. Both the initial investment and the operating costs of the system must be taken into consideration. However, environmental and technical aspects cannot be quantified. Therefore subjective rankings must be used. Table 24 indicates that aerobic ponds is the least desirable method of concentrated wastewater treatment in places were productive land is to be used for construction of the ponds. Anaerobic lagoons are the least expensive and are used more often than any other treatment in the management of wastewater from feedlots. However, they are not recommended as a permanent solution.
|
Treatment |
Initial |
Operating |
Land |
Energy |
Ecology |
Index |
|
Type |
cost indexb |
cost indexb |
area indexb |
rankingc |
rankingc |
rankd |
|
1: Anaerobic lagoonsa |
1 |
1 |
20 |
1 |
6 |
1 |
|
2: Aerobic ponds |
6 |
4 |
300 |
2 |
5 |
20 |
|
3: Aerated lagoonsa |
6 |
13 |
3 |
4 |
4 |
2 |
|
4: Oxidation ditches |
8 |
16 |
9 |
3 |
3 |
|
|
5: Physical/biological |
25 |
40 |
10 |
5 |
2 |
6 |
|
6: Physical/biological/chemical |
50 |
70 |
2 |
6 |
1 |
10 |
a Exclusive of land acquisition costs. It is assumed that land used in the construction of the treatment plant is owned by the feedlot.b Index is the ratio of the treatment cost to that of the least cost treatment. Thus, the least cost treatment would have an index of 1. An index of 6 means 6 times more expensive than the least cost treatment in that category.
c Ranking is a judgement ranking of the six potential systems ranked in order of preference from 1 to 6. The ranking is not on the basis of cost, nor does a ranking of 6 means it is 6 times less diserable than that ranked 1 in the same category.
d Index/rank is a combination of cost rations and judgement rankings reflecting the author’s preference based on technical, economic, and ecological feasibility of the system.
Prevention of waste production, as a method mentioned for solid waste and wastewater, is an even more important method for polluted air. Compared to solid waste and wastewater, awareness of air pollution has only recently developed. As a result data of produced polluted air are hardly available and methods for prevention or treatment have as yet not been developed on a wide scale. Research of methods for treatment of polluted air is in progress.
Available reports state that air pollution from red meat processing operations is minor, and that most problems usually involve odours from improper waste treatment. These had better be controlled by correction of deficiencies in the troublesome process than by attempts to treat the odorous air. Also for poultry plants is true that a good design and good operating practices may help prevent odours.
In the dairy industry much energy is required for all kinds of activities and in particular for cooling and heating (pasteurizing, sterilizing, vaporisation). Energy production may lead to excessive discharge of CO2, NOx and CO. Reduction of energy consumption leads to an decrease of the discharge of these gases.
Air pollution from tanneries and slaughterhouses may also be caused by such components as NH3, SO2, Volatile Organic Compounds (VOC), etc. These can in principle be removed by means of adsorption, absorption, chemical reactions, microbial conversion. These processes often require highly sophisticated technics and entail high costs.
Adsorption is a process in which gasses are adsorbed to solid material such as carbon. In the case of absorption a liquid is used. Usually these physical processes are combined with a chemical conversion process to fixate the polluting compound. Other chemical reactions involve oxidation at high or low temperatures, reduction with hydrogen or methane. Biodegradable compounds can be converted by micro-organisms, suspended in water or fixated on solid material such as compost.
