ABSTRACT

The Traiselect system is based on the idea to treat black wastewaters and grey wastewaters on a selective manner. In one of the versions of this system, according to the use of a dry toilet, black wastewaters are not produced, only the grey waters have to be treated.

This study has been carried through 6 installations located in Wallonia (South region of Belgium) where only the grey waters were treated and using dry toilets, TBL toilets (toilets with biocontrolled litter) of which more than 600 equipments are already operational in private houses in Belgium and in France. The method and advantages of the TBL will be described in this report.

When our study was carried through, the equipments were 2 or 4 years old; a monthly sampling has been carried out during a period of one year.

Further to the analysis of the average values of the whole Traiselect installations we can notice that for the main sanitation parameters (BOD, COD, SS, NH₄, N_org), the exhaustion rates are above 90 %, only phosphorus has a reduction of about 50 %.

The non production of black wastewaters (which traditionally constitute more than 30 % of the domestic effluents in Wallonia) allow us to understand that the hydrous and nitrogenous rejects of these installations are really impressive compared with the classical sanitation systems.

In conclusion, these equipments not only meet the imposed rejection standards but their nitrogenous results and their environmental impacts are very below compared to the classical best treatment systems.

The use of a dry toilet, the TBL toilet, considerably reduces the quantity of nitrates and phosphorous rejected on a mineralized shape in the receiving area.

Unfortunately the legislation in force today in Wallonia does not encourage this procedure which however constitutes a perfect example of an integrated system.

1. The conventional view of sewage treatment

Most autonomous sewage treatment systems on the market are actually designed to achieve a single purpose, that of sanitizing household sewage as well as possible by oxidizing its organic content so that the latter can be mineralized. Thus, organic nitrogen is turned into the ions NH₄^-, NO₂^-, and NO₃^-, which are all highly soluble in water, while organic phosphorus is broken down to yield PO₄^-3. Since all of these ions are undesirable in the receiving environment, the next step is to remove them from the treated water. In conventional treatment schemes, most of the nitrogen and phosphorus that enter the installation will be trapped in the sludge. After the sludge is spread on farmland part of the released nitrate ions enter the ground water and another part the surface water. A variable but usually small percentage of the nitrogen will be taken up by the plants. In this way, the treated water disrupts the receiving environment to various degrees, especially when the latter is a sluggish stream. When the treated water percolates through the ground its environmental impact is lessened, although one still must allow for the problems created by the treatment sludge.

Classically, an autonomous treatment system is considered ‘good’ when the nitrogen and phosphorus’s mineralization is very complete, even though the mineralized species of nitrogen and, to a lesser extent, phosphorus, eventually end up in the environment’s natural water system. In evaluating the various processes one must also allow for the energy that is expended to treat, transport, and dispose of the sludge.

2. A new approach

Article 1 of EC Directive 271/91 gives a good definition of the ultimate purpose of sewage treatment, i.e., protecting the environment as best as one can. Given the ease with which the mineralized species of nitrogen migrate and dissolve, treating waste well does not necessarily achieve this end and can even be counterproductive. To Asses a treatment system’s performance we thus propose replacing the notion of ‘treatment performance’ with that of ‘environmental performance’. Instead of treating as best one can, the aim becomes that of minimizing environmental impacts up- and downstream from the treatment operation. This vision of things was the starting point for developing the TRAISELECT system (from the French for “selective treatment”) for domestic sewage that is the focus of this work. What is more, once you give up the idea that the composition of the sewage to treat is an immutable given, new recovery prospects open up.

3. Black water and grey water

Analysis of the treatment systems currently on the market reveals two things:

They have to treat a mixture of wastewater resulting from the entire gamut of household activities.
Approximately 80% of the wastewater’s nitrogen content comes from the toilets and urinals.⁹

The fact that “black water” (sewage from toilets) and “grey water” (the wastewater produced by other household activities) are treated together creates a true engineering challenge, while mixing these two categories of sewage reduces the treatment’s effectiveness when it comes to protecting the environment into which the cleaned water and sludge are discharged.

For better environmental protection, the first step is to examine the possibilities of changing water use habits. The next step is to install the treatment system that has the smallest environmental impact. To do this, we must examine the composition of the wastewater that is produced by the various activities in the home. Thus, black water accounts for 15-35% of the volume of domestic sewage.^1,
9 Its pollutant load consists primarily of proteins containing nitrogen and phosphorus, as well as a certain amount of urea. Through his or her excrement, each individual discharges between 5 and 9 kg of nitrogen and slightly less than 1 kg of phosphorus into the environment each year. Grey water, while accounting for much greater volumes, contains very little nitrogen,⁸ while its phosphorus content can be practically nil if phosphate-free detergents are used. The problem can thus be simplified as follows: on one side we have a little water containing almost all of the nitrogen and phosphorus, and on the other side we have a lot of water that contains only soap and detergent. Aerobic treatment of the latter will yield only water, carbon dioxide, and a series of ions (SO₄^-, Cl^-, and CO₃^-) that are environmentally neutral.

4. The TRAISELECT treatment system

The TRAISELECT system is based on the idea of treating black and grey water selectively. The system comes in two versions. One of them treats the water discharged from a low-flush toilet alongside the treatment of grey water. The other treats grey water only. In the latter case, no black water is produced, thanks to the use of a dry toilet. The study described in this article was performed on six installations that treated grey water only and used consequently dry toilets.

5. Is it possible not to produce black water?

The answer to this question is “yes,” provided that the user is willing to replace her/his flush toilet with an appropriate dry toilet. This is the most direct application of the principle of preventing pollution at its source. Indeed, the bulk of domestic wastewater problems is due to the presence of nitrogen in the sewage from our toilets. The metabolic phosphorus is a problem only if the treated water is discharged into surface waters. By preventing the production of black water one effectively ‘pulls the plug’ on the majority of the problems linked to domestic sanitation. In rural and peri-urban areas one can say without exaggerating that preventing the production of black water is the key to controlling domestic sources of water pollution at little cost.

5.1 “Swedish” toilets

Most of the systems on the market separate the liquid and solids for subsequent separate handling of the various effluents. The water savings achieved by using these toilets are often touted. Because of their potential foul odors and high prices, their use is limited to areas that are not hooked up to a water supply network.

Separating the urine and feces is conception responsible for the problem of smell and requires the installation of forced ventilation systems that are of variable effectiveness but always expensive. This also has an environmental impact, that of releasing the urinary nitrogen (60-80% of the excreted nitrogen) in the form of ammonia. Indeed, after 24 hours’ storage, some 90%of the organic nitrogen in the urine has turned into ammonia. Once that happens, recovering the nitrogen to contribute to the humus layer’s formation becomes problematic. On the other hand, letting the urine trickle into the ground is an inadmissible form of pollution, never mind the fact that the nitrogen that has been turned into pollution is lost to the biosphere.

5.2 The biocontrolled composting toilet (BCT)

The development of this new type of dry toilet enabled us to envision the selective treatment of residential wastewater production realistically. This toilet was the first step in producing a device that could be used without reducing modern levels of comfort noticeably. What is more, it channels the organic nitrogen and phosphorus into the ground in the form of humus without any losses to the groundwater.

The idea of a bedding toilet is a by-product of the search for idea conditions for recycling human and animal manure as humus to regenerate farmland. The aim is not to turn the effluents into fertilizers, although this effect cannot be discounted. Acid humus is formed, under well-defined conditions, from a set of nitrogen- and carbon-containing organic compounds from the system’s biomass. The starting mixture must have a C/N (carbon-to-nitrogen) ratio of at least 60 for the humus to be synthesized. Animal and human manure have a C/N ratio of about 7; they are thus too rich in nitrogen. If they are discharged as is into nature they will release a large amount of leachable nitrogen into the environment, unless sufficient carbon substrate of plant origin is available. This explains how the water table is polluted by liquid manure, manure, and even poorly prepared compost. In no event may an organic agricultural fertilizer smell of ammonia when it is being spread. Even if plant material is added to the stored excrement, most of the ammonia that is released during the storage is no longer available for soil formation and is potentially polluting. The excrement’s C/N ratio must be adjusted as soon as they are produced in order to inhibit urease’s action and the formation of ammonia.³

Through this same mechanism, adjusting the excrement’s C/N ratio as soon as it is produced also controls the release of odors. This was the starting point for designing the biocontrolled composting toilet.

Technically speaking, the application of this discovery is both simple and complex. The idea is to use plant biomass rather than water to get rid of human excrement. Besides its very favorable environmental life cycle analysis, this toilet has the following major advantage: The simple elimination of all odor means that it may be placed in a dwelling without adding an extraction fan. This reduces its installation cost greatly.

The approximately 600 bedding toilets that were already operating in private homes in Belgium and France in January 2003 produce composted human manure in the family gardens, even in cities. An epidemiological survey⁴ of the users has revealed the harmlessness of this option. The composting can be done under acceptable sanitary conditions.

The current users share the following socio-demographic characteristic: a level of education well above the mean for the population. A sociological survey⁶ has revealed that the families – who are often well off – who managed to overcome the psychological hurdles to using a compost toilet and adopted the BCT are adamantly opposed to going back to using a flush toilet.

6. The TRAISELECT installations that we studied

We studied six selective greywater treatment installations located in the following towns in Belgium: Leers-Nord (Tournai), Mons, Epinois (Binche), Glabais, Malonne, and Louvain-la-Neuve.

These installations are from 2 to 4 years old. We can thus consider them to have reached a stable operating regime from a microbiological (ecological) point of view. The relative diversity of the installations and how they are used is explained by the following factors:

- The installations were installed by volunteer households using their own money, without aid or subsidies.

- Depending on the case, the installations were either planned at the time of the dwelling’s construction or installed in existing dwellings.

- Despite certain instructions for their use, the systems’ “operating conditions” were not identical. For example, four of the six households used rainwater only, whereas the others used piped water. This explains some of the differences in the very different degrees of mineralization of the wastewater (from 472 to 1374 µS/cm).

- The use of variable amounts and types of cleaning products, e.g., laundry detergents with and without phosphates.

Given these conditions, the aim of monitoring these “pilot” installations was neither to determine their best performances nor to quantify the various processes that are involved in their treatment abilities. The main objective was to show, by means of a series of analyses, that under real operating conditions these installations were able to meet interesting treatment targets compatible with the aims of protecting the receiving environment.

7. Effluent quality

7.1. Methods

Samples were taken at regular intervals from all of the installations according to an identical protocol. So, between 15 October 1999 and 9 August 2000 each installation was sampled at two points in the circuit (upstream from the aerobic reactor and aeration tank (= “inlet value”) and downstream from the installation before effluent discharge into the receiving environment (= “outlet value”)) once a month. The representativeness of these spot checks was guaranteed by the installations’ large buffer volumes.

The samples were refrigerated as soon as they were collected, then analyzed by a Walloon Region Administration-certified laboratory (RH Laboratory, FUL, in Arlon) within 24 hours. The analyses were performed on the crude samples (neither settled nor filtered) using the conventional standardized methods.^¹0 Table 2 gives the ten-round averages for the twelve parameters that we investigated. The complete results of these observations (six tables containing the ten series of monthly observations) may be consulted by those wishing to do so at the following address: jozsef.orszagh@skynet.be.

Table 1. Mean values for 6 TRAISELECT installations.

8. Comments

First of all, one must bear in mind that the values in table 1 are the averages of ten monthly measurements. These data thus have a significant statistical value that includes any fluctuations that might have occurred because of fluctuations in use and weather conditions.

Analysis of the mean values for all of the TRAISELECT installations (Column 7) shows that the abatement rates for the main treatment parameters (BOD₅, COD, and dry matter^ⁱ) were greater than 90%. The same goes for the ammonia nitrogen and organic nitrogen levels. In contrast, the abatement in the phosphorus level (orthophosphate and total phosphate) was only of the order of 50%.

We must point out that these yields were obtained from “inlet values” that corresponded, as mentioned earlier to an intermediate site in the circuit for which abatement had already been achieved, notably by allowing sediment to settle in the unstirred tank upstream from the measuring point (e.g., the septic tank). We made this sampling choice deliberately so as to take advantage of the hydraulic volume to offset the variations in concentration. However, this “underestimation” of the actual inlet concentration resulted in a corresponding underestimation of the installations’ actual recovery rates.

What is most important in monitoring these “pilot” installations is to notice that in all cases the concentrations measured leaving the treatment installations are always well below the sectoral conditions of the home treatment systems.⁷ So, even if we consider the worst conditions seen during this annual monitoring cycle (Column 9 = maximum), none of these sectoral emissions conditions was ever exceeded, far from it (COD = 116 versus 180; BOD = 22 versus 70; and SS = 29 versus 60). Turning to the nitrogen species, we see that 25% of the organic nitrogen was eliminated along with 90% of the ammonia nitrogen. If these transformations involve nitrification of the nitrogen, the nitrate concentration was not increased significantly. This means that very effective denitrification can take place in the system, given the Traiselect’s particularities.

Cumulative load

We also measured the load discharged by Installation 2, which was a 3 EH installation with a daily throughput of 180 liters. The total nitrogen discharged was N_tot = N_NO3 + N_K = 0.21 + 2.8 = 3.01 mg N/liter. This corresponds to 542 mg N/day = 198 gm N/yr for the installation or a discharge of 66 gm N per year per person. The phosphorus load amounted to P_tot = 0.12 mg P/liter = 21.6 mg P/day = 7.88 gm P/yr for the installation and 2.63 gm of phosphorus discharged per year per person into the receiving environment.

Sludge production

We measured the amount of sludge produced in Installation 2’s 2-m³ anaerobic reactor after 480 days of operation, during which 86.4 m³ of grey water was treated. After draining off the water we collected 172 liters of sludge at the bottom of the reactor. This sludge contained 33.1 gm of dry matter per liter, giving a total of 5.9 kg. The characteristics of this sludge are summarized in the table below:

Characteristics of greywater treatment sludge
Organic matter (gm/100 gm dry matter)	49
TOC (gm/100 gm dry matter)	8.65
N_tot (gm/100 gm dry matter)	3.04
P_tot (gm/100 gm dry matter)	0.83
C/N	2.8

Given that the installation does not produce any black water, which on average accounts for 30-40% of domestic effluent in Wallonia, the benefits of such a treatment system are clear.

9. Conclusions and future prospects

The main aim of these installations was met, for the outlet concentrations of organic matter, N, and P are remarkably in line with the discharge criteria imposed on home treatment installations.⁽⁷⁾

Using a dry toilet such as the biocontrolled composting toilet (BCT) reduces considerably the amounts of mineralized nitrogen and phosphorus discharged into the receiving environment. Using this new type of toilet offers such a level of comfort that it is not unreasonable to consider its being extended to use in the home. The non-production of black water reduces the environmental impact of domestic sewage considerably. The selective treatment of the grey water can reduce the cost of sewage treatment investments in residential areas greatly. The TRAISELECT system, along with other treatment schemes using green plants, is one of the new elements of the “integrated sewage treatment” concept.⁸

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